hi,

as discussed on irc, i propose to change the histedit command `mesg`
to behave like the `pick` command but to edit the commit message in
$EDITOR. this is how that would look like:

```
drop <commit>
fold <commit>
# same as `pick` but edit log message in $EDITOR
mesg <commit>
```

these are the reasons for this change:
- having fold operate on the next commit and mesg on the previous one is
  confusing. there is no good place to put mesg
- probably none is using the scripting functionality of mesg outside of
  the regression tests. you can still use a editor.sh script
- it is more intuitive to use `mesg` like `pick`

the regress tests are passing for me. as expected, i had to add some
editor.sh "hacks".

diff /home/lorenz/src/got
commit - 41208069f572694ef8847dbe9d9a47337f7083bd
path + /home/lorenz/src/got
blob - 52eeeda8df9fb52a8581faa28c2d29c19ca5a2be
file + got/got.1
--- got/got.1
+++ got/got.1
@@ -3016,18 +3016,7 @@ Regular editing of history must eventually be resumed 
 .It Cm fold Ar commit Ta Combine the specified commit with the next commit
 listed further below that will be used.
 .It Cm drop Ar commit Ta Remove this commit from the edited history.
-.It Cm mesg Oo Ar log-message Oc Ta Create a new log message for the commit of
-a preceding
-.Cm pick
-or
-.Cm edit
-command on the previous line of the histedit script.
-The optional
-.Ar log-message
-argument provides a new single-line log message to use.
-If the
-.Ar log-message
-argument is omitted, open an editor where a new log message can be written.
+.It Cm mesg Ar commit Ta Use the specified commit but edit the log message.
 .El
 .Pp
 Every commit in the history being edited must be mentioned in the script.
blob - 73812ddbb0f2e3a0fee927fc0ae5b873c04e385a
file + got/got.c
--- got/got.c
+++ got/got.c
@@ -11696,8 +11696,7 @@ static const struct got_histedit_cmd {
 	{ GOT_HISTEDIT_FOLD, "fold", "combine with next commit that will "
 	    "be used" },
 	{ GOT_HISTEDIT_DROP, "drop", "remove commit from history" },
-	{ GOT_HISTEDIT_MESG, "mesg",
-	    "single-line log message for commit above (open editor if empty)" },
+	{ GOT_HISTEDIT_MESG, "mesg", "use commit but edit the log message" },
 };
 
 struct got_histedit_list_entry {
@@ -11760,16 +11759,11 @@ histedit_write_commit_list(struct got_object_id_queue 
 			histedit_cmd = "edit";
 		else if (fold_only && STAILQ_NEXT(qid, entry) != NULL)
 			histedit_cmd = "fold";
+		else if (edit_logmsg_only)
+			histedit_cmd = "mesg";
 		err = histedit_write_commit(&qid->id, histedit_cmd, f, repo);
 		if (err)
 			break;
-		if (edit_logmsg_only) {
-			int n = fprintf(f, "%c\n", GOT_HISTEDIT_MESG);
-			if (n < 0) {
-				err = got_ferror(f, GOT_ERR_IO);
-				break;
-			}
-		}
 	}
 
 	return err;
@@ -11986,7 +11980,7 @@ histedit_parse_list(struct got_histedit_list *histedit
 	char *line = NULL, *p, *end;
 	size_t i, linesize = 0;
 	ssize_t linelen;
-	int lineno = 0, lastcmd = -1;
+	int lineno = 0;
 	const struct got_histedit_cmd *cmd;
 	struct got_object_id *commit_id = NULL;
 	struct got_histedit_list_entry *hle = NULL;
@@ -12026,37 +12020,15 @@ histedit_parse_list(struct got_histedit_list *histedit
 		}
 		while (isspace((unsigned char)p[0]))
 			p++;
-		if (cmd->code == GOT_HISTEDIT_MESG) {
-			if (lastcmd != GOT_HISTEDIT_PICK &&
-			    lastcmd != GOT_HISTEDIT_EDIT) {
-				err = got_error(GOT_ERR_HISTEDIT_CMD);
-				break;
-			}
-			if (p[0] == '\0') {
-				err = histedit_edit_logmsg(hle, repo);
-				if (err)
-					break;
-			} else {
-				hle->logmsg = strdup(p);
-				if (hle->logmsg == NULL) {
-					err = got_error_from_errno("strdup");
-					break;
-				}
-			}
-			lastcmd = cmd->code;
-			continue;
-		} else {
-			end = p;
-			while (end[0] && !isspace((unsigned char)end[0]))
-				end++;
-			*end = '\0';
-
-			err = got_object_resolve_id_str(&commit_id, repo, p);
-			if (err) {
-				/* override error code */
-				err = histedit_syntax_error(lineno);
-				break;
-			}
+		end = p;
+		while (end[0] && !isspace((unsigned char)end[0]))
+			end++;
+		*end = '\0';
+		err = got_object_resolve_id_str(&commit_id, repo, p);
+		if (err) {
+			/* override error code */
+			err = histedit_syntax_error(lineno);
+			break;
 		}
 		hle = malloc(sizeof(*hle));
 		if (hle == NULL) {
@@ -12068,7 +12040,12 @@ histedit_parse_list(struct got_histedit_list *histedit
 		hle->logmsg = NULL;
 		commit_id = NULL;
 		TAILQ_INSERT_TAIL(histedit_cmds, hle, entry);
-		lastcmd = cmd->code;
+
+		if (cmd->code == GOT_HISTEDIT_MESG) {
+			err = histedit_edit_logmsg(hle, repo);
+			if (err)
+				break;
+		}
 	}
 
 	free(line);
@@ -12270,15 +12247,6 @@ histedit_save_list(struct got_histedit_list *histedit_
 		    repo);
 		if (err)
 			break;
-
-		if (hle->logmsg) {
-			int n = fprintf(f, "%c %s\n",
-			    GOT_HISTEDIT_MESG, hle->logmsg);
-			if (n < 0) {
-				err = got_ferror(f, GOT_ERR_IO);
-				break;
-			}
-		}
 	}
 done:
 	if (f && fclose(f) == EOF && err == NULL)
@@ -12416,6 +12384,7 @@ show_histedit_progress(struct got_commit_object *commi
 	switch (hle->cmd->code) {
 	case GOT_HISTEDIT_PICK:
 	case GOT_HISTEDIT_EDIT:
+	case GOT_HISTEDIT_MESG:
 		printf("%s -> %s: %s\n", old_id_str,
 		    new_id_str ? new_id_str : "no-op change", logmsg);
 		break;
blob - a8c20c791a546fc257c69e471cb4ff9e59cd4d66
file + regress/cmdline/histedit.sh
--- regress/cmdline/histedit.sh
+++ regress/cmdline/histedit.sh
@@ -491,13 +491,22 @@ test_histedit_fold() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/committing folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	echo "fold $old_commit1" > $testroot/histedit-script
 	echo "drop $old_commit2" >> $testroot/histedit-script
 	echo "pick $old_commit3" >> $testroot/histedit-script
-	echo "mesg committing folded changes" >> $testroot/histedit-script
 
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout)
 
 	local new_commit1=`git_show_parent_commit $testroot/repo`
 	local new_commit2=`git_show_head $testroot/repo`
@@ -603,7 +612,6 @@ test_histedit_edit() {
 	fi
 
 	echo "edit $old_commit1" > $testroot/histedit-script
-	echo "mesg committing changes" >> $testroot/histedit-script
 	echo "pick $old_commit2" >> $testroot/histedit-script
 
 	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
@@ -650,8 +658,20 @@ test_histedit_edit() {
 	fi
 
 	(cd $testroot/wt && got unstage alpha > /dev/null)
-	(cd $testroot/wt && got histedit -c > $testroot/stdout)
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/committing changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -c > $testroot/stdout)
+
 	local new_commit1=`git_show_parent_commit $testroot/repo`
 	local new_commit2=`git_show_head $testroot/repo`
 
@@ -771,7 +791,7 @@ test_histedit_fold_last_commit() {
 	test_done "$testroot" "$ret"
 }
 
-test_histedit_missing_commit() {
+test_histedit_missing_commit_pick() {
 	local testroot=`test_init histedit_missing_commit`
 
 	local orig_commit=`git_show_head $testroot/repo`
@@ -795,7 +815,6 @@ test_histedit_missing_commit() {
 	fi
 
 	echo "pick $old_commit1" > $testroot/histedit-script
-	echo "mesg committing changes" >> $testroot/histedit-script
 
 	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
 		> $testroot/stdout 2> $testroot/stderr)
@@ -818,6 +837,63 @@ test_histedit_missing_commit() {
 	test_done "$testroot" "$ret"
 }
 
+test_histedit_missing_commit_mesg() {
+	local testroot=`test_init histedit_missing_commit`
+
+	local orig_commit=`git_show_head $testroot/repo`
+
+	echo "modified alpha on master" > $testroot/repo/alpha
+	git -C $testroot/repo rm -q beta
+	echo "new file on master" > $testroot/repo/epsilon/new
+	git -C $testroot/repo add epsilon/new
+	git_commit $testroot/repo -m "committing changes"
+	local old_commit1=`git_show_head $testroot/repo`
+
+	echo "modified zeta on master" > $testroot/repo/epsilon/zeta
+	git_commit $testroot/repo -m "committing to zeta on master"
+	local old_commit2=`git_show_head $testroot/repo`
+
+	got checkout -c $orig_commit $testroot/repo $testroot/wt > /dev/null
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		test_done "$testroot" "$ret"
+		return 1
+	fi
+
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/committing folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
+	echo "mesg $old_commit1" > $testroot/histedit-script
+
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout \
+			2>$testroot/stderr)
+
+	ret=$?
+	if [ $ret -eq 0 ]; then
+		echo "histedit succeeded unexpectedly" >&2
+		test_done "$testroot" "1"
+		return 1
+	fi
+
+	echo "got: commit $old_commit2 missing from histedit script" \
+		> $testroot/stderr.expected
+
+	cmp -s $testroot/stderr.expected $testroot/stderr
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		diff -u $testroot/stderr.expected $testroot/stderr
+	fi
+	test_done "$testroot" "$ret"
+}
+
 test_histedit_abort() {
 	local testroot=`test_init histedit_abort`
 
@@ -845,7 +921,6 @@ test_histedit_abort() {
 	touch $testroot/wt/unversioned-file
 
 	echo "edit $old_commit1" > $testroot/histedit-script
-	echo "mesg committing changes" >> $testroot/histedit-script
 	echo "pick $old_commit2" >> $testroot/histedit-script
 
 	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
@@ -1051,7 +1126,6 @@ test_histedit_path_prefix_edit() {
 	fi
 
 	echo "edit $old_commit1" > $testroot/histedit-script
-	echo "mesg modified zeta" >> $testroot/histedit-script
 
 	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
 		> $testroot/stdout 2> $testroot/stderr)
@@ -1120,8 +1194,19 @@ test_histedit_path_prefix_edit() {
 		test_done "$testroot" "$ret"
 		return 1
 	fi
+	
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/modified zeta/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
 
-	(cd $testroot/wt && got histedit -c > $testroot/stdout)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -c > $testroot/stdout)
 
 	local new_commit1=`git_show_head $testroot/repo`
 	local short_new_commit1=`trim_obj_id 28 $new_commit1`
@@ -1244,13 +1329,23 @@ test_histedit_fold_last_commit_swap() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/committing folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	# fold commit2 into commit1 (requires swapping commits)
 	echo "fold $old_commit2" > $testroot/histedit-script
-	echo "pick $old_commit1" >> $testroot/histedit-script
-	echo "mesg committing folded changes" >> $testroot/histedit-script
+	echo "mesg $old_commit1" >> $testroot/histedit-script
 
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout 2> $testroot/stderr)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout \
+		2> $testroot/stderr)
 
 	ret=$?
 	if [ $ret -ne 0 ]; then
@@ -1470,13 +1565,22 @@ test_histedit_fold_add_delete() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	echo "fold $old_commit1" > $testroot/histedit-script
 	echo "fold $old_commit2" >> $testroot/histedit-script
 	echo "pick $old_commit3" >> $testroot/histedit-script
-	echo "mesg folded changes" >> $testroot/histedit-script
 
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout)
 
 	local new_commit1=`git_show_head $testroot/repo`
 
@@ -1565,12 +1669,21 @@ test_histedit_fold_edit_delete() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	echo "fold $old_commit1" > $testroot/histedit-script
 	echo "pick $old_commit2" >> $testroot/histedit-script
-	echo "mesg folded changes" >> $testroot/histedit-script
 
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout)
 
 	local new_commit1=`git_show_head $testroot/repo`
 
@@ -1645,12 +1758,21 @@ test_histedit_fold_delete_add() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/folded changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	echo "fold $old_commit1" > $testroot/histedit-script
 	echo "pick $old_commit2" >> $testroot/histedit-script
-	echo "mesg folded changes" >> $testroot/histedit-script
 
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout)
+	(cd $testroot/wt && env EDITOR="$testroot/editor.sh" \
+		VISUAL="$testroot/editor.sh" \
+		got histedit -F $testroot/histedit-script > $testroot/stdout)
 
 	local new_commit1=`git_show_head $testroot/repo`
 	
@@ -2142,126 +2264,6 @@ EOF
 	test_done "$testroot" $ret
 }
 
-test_histedit_mesg_invalid() {
-	local testroot=`test_init mesg_invalid`
-
-	local orig_commit=`git_show_head $testroot/repo`
-
-	echo "modified alpha on master" > $testroot/repo/alpha
-	git -C $testroot/repo rm -q beta
-	echo "new file on master" > $testroot/repo/epsilon/new
-	git -C $testroot/repo add epsilon/new
-	git_commit $testroot/repo -m 'committing changes'
-	local old_commit1=`git_show_head $testroot/repo`
-
-	echo "modified zeta on master" > $testroot/repo/epsilon/zeta
-	git_commit $testroot/repo -m 'committing to zeto on master'
-	local old_commit2=`git_show_head $testroot/repo`
-
-	got checkout -c $orig_commit $testroot/repo $testroot/wt > /dev/null
-	ret=$?
-	if [ $ret -ne 0 ]; then
-		test_done "$testroot" $ret
-		return 1
-	fi
-
-	# try with a leading mesg
-
-	echo "mesg something something" > $testroot/histedit-script
-	echo "pick $old_commit1" >> $testroot/histedit-script
-	echo "pick $old_commit2" >> $testroot/histedit-script
-
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout 2> $testroot/stderr)
-	ret=$?
-	if [ $ret -eq 0 ]; then
-		echo "histedit succeeded unexpectedly" >&2
-		test_done "$testroot" 1
-		return 1
-	fi
-
-	echo "got: bad histedit command" > $testroot/stderr.expected
-	cmp -s $testroot/stderr.expected $testroot/stderr
-	ret=$?
-	if [ $ret -ne 0 ]; then
-		diff -u $testroot/stderr.expected $testroot/stderr
-		test_done "$testroot" $ret
-		return 1
-	fi
-
-	# try again with mesg -> mesg
-
-	echo "pick $old_commit1" > $testroot/histedit-script
-	echo "mesg something something" >> $testroot/histedit-script
-	echo "mesg something something else" >> $testroot/histedit-script
-	echo "pick $old_commit2" >> $testroot/histedit-script
-
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout 2> $testroot/stderr)
-	ret=$?
-	if [ $ret -eq 0 ]; then
-		echo "histedit succeeded unexpectedly" >&2
-		test_done "$testroot" 1
-		return 1
-	fi
-
-	echo "got: bad histedit command" > $testroot/stderr.expected
-	cmp -s $testroot/stderr.expected $testroot/stderr
-	ret=$?
-	if [ $ret -ne 0 ]; then
-		diff -u $testroot/stderr.expected $testroot/stderr
-		test_done "$testroot" $ret
-		return 1
-	fi
-
-	# try again with drop -> mesg
-
-	echo "drop $old_commit1" > $testroot/histedit-script
-	echo "mesg something something" >> $testroot/histedit-script
-	echo "pick $old_commit2" >> $testroot/histedit-script
-
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout 2> $testroot/stderr)
-	ret=$?
-	if [ $ret -eq 0 ]; then
-		echo "histedit succeeded unexpectedly" >&2
-		test_done "$testroot" 1
-		return 1
-	fi
-
-	echo "got: bad histedit command" > $testroot/stderr.expected
-	cmp -s $testroot/stderr.expected $testroot/stderr
-	ret=$?
-	if [ $ret -ne 0 ]; then
-		diff -u $testroot/stderr.expected $testroot/stderr
-		test_done "$testroot" $ret
-		return 1
-	fi
-
-	# try again with fold -> mesg
-
-	echo "fold $old_commit1" > $testroot/histedit-script
-	echo "mesg something something" >> $testroot/histedit-script
-	echo "pick $old_commit2" >> $testroot/histedit-script
-
-	(cd $testroot/wt && got histedit -F $testroot/histedit-script \
-		> $testroot/stdout 2> $testroot/stderr)
-	ret=$?
-	if [ $ret -eq 0 ]; then
-		echo "histedit succeeded unexpectedly" >&2
-		test_done "$testroot" 1
-		return 1
-	fi
-
-	echo "got: bad histedit command" > $testroot/stderr.expected
-	cmp -s $testroot/stderr.expected $testroot/stderr
-	ret=$?
-	if [ $ret -ne 0 ]; then
-		diff -u $testroot/stderr.expected $testroot/stderr
-	fi
-	test_done "$testroot" $ret
-}
-
 test_histedit_resets_committer() {
 	local testroot=`test_init histedit_resets_committer`
 	local orig_commit=`git_show_head $testroot/repo`
@@ -2354,13 +2356,22 @@ test_histedit_umask() {
 		return 1
 	fi
 
+	cat > $testroot/editor.sh <<EOF
+#!/bin/sh
+ed -s "\$1" <<-EOF
+	,s/.*/folding changes/
+	w
+	EOF
+EOF
+	chmod +x $testroot/editor.sh
+
 	echo fold $commit1 >$testroot/histedit-script
 	echo fold $commit2 >>$testroot/histedit-script
 	echo pick $commit3 >>$testroot/histedit-script
-	echo mesg folding changes >>$testroot/histedit-script
 
 	# using a subshell to avoid clobbering global umask
 	(umask 077 && cd "$testroot/wt" && \
+		env EDITOR="$testroot/editor.sh" VISUAL="$testroot/editor.sh" \
 		got histedit -F "$testroot/histedit-script") >/dev/null
 	ret=$?
 
@@ -2604,7 +2615,8 @@ run_test test_histedit_drop
 run_test test_histedit_fold
 run_test test_histedit_edit
 run_test test_histedit_fold_last_commit
-run_test test_histedit_missing_commit
+run_test test_histedit_missing_commit_pick
+run_test test_histedit_missing_commit_mesg
 run_test test_histedit_abort
 run_test test_histedit_path_prefix_drop
 run_test test_histedit_path_prefix_edit
@@ -2619,7 +2631,6 @@ run_test test_histedit_fold_only
 run_test test_histedit_fold_only_empty_logmsg
 run_test test_histedit_edit_only
 run_test test_histedit_prepend_line
-run_test test_histedit_mesg_invalid
 run_test test_histedit_resets_committer
 run_test test_histedit_umask
 run_test test_histedit_mesg_filemode_change