Unless commit -F was used, it should be possible to abort a commit
operation by exiting the log message editor without making any
modifications to the log message.

This currently does not work as expected in the presence of our new
cherrypic/backout log messages, because the prepared_log variable
was overloaded for this purpose. Use a different variable, and add
merged log messages to the initial log message content buffer.
This makes it possible to abort commits as expected.

Erroring out if merged messages are not modified is probably a good
thing in any case. I suspect that in virtually all cases the merged
log messages should be edited, because the reason for cherrypick or
backout should be explained.

A related question is whether merged messages should be added to the
initial content even if -F is used. Currently, they are not added and
will be discarded. My patch keeps this behaviour as-is.
I am unsure about this. Users will probably expect that -F messages
are left as-is. However, merged log messages will be discarded and the
user might be unaware of their presence.
Let's hope if someone uses -F, they really mean -F?

ok?
 
 abort commit with merged log messages if the log message is left unmodified
 
diff c7ee7a0aebd0cf0ba96bd4215bd263d8f1dd8e6b b026395ef9db83a7e0f92424b0d01c5016e9fb36
commit - c7ee7a0aebd0cf0ba96bd4215bd263d8f1dd8e6b
commit + b026395ef9db83a7e0f92424b0d01c5016e9fb36
blob - 1ebe8e6bc34a5cf56e0824f9c4b8c0f986f38301
blob + 872cc74858a6d9a7d5be7fffffe97fc4f28c9927
--- got/got.c
+++ got/got.c
@@ -8725,6 +8725,7 @@ struct collect_commit_logmsg_arg {
 struct collect_commit_logmsg_arg {
 	const char *cmdline_log;
 	const char *prepared_log;
+	const char *merged_log;
 	int non_interactive;
 	const char *editor;
 	const char *worktree_path;
@@ -8791,6 +8792,7 @@ collect_commit_logmsg(struct got_pathlist_head *commit
 	struct got_pathlist_entry *pe;
 	const struct got_error *err = NULL;
 	char *template = NULL;
+	char *prepared_msg = NULL, *merged_msg = NULL;
 	struct collect_commit_logmsg_arg *a = arg;
 	int initial_content_len;
 	int fd = -1;
@@ -8815,21 +8817,19 @@ collect_commit_logmsg(struct got_pathlist_head *commit
 		goto done;
 
 	if (a->prepared_log) {
-		char *msg;
-		err = read_prepared_logmsg(&msg, a->prepared_log);
+		err = read_prepared_logmsg(&prepared_msg, a->prepared_log);
 		if (err)
 			goto done;
-		if (write(fd, msg, strlen(msg)) == -1) {
-			err = got_error_from_errno2("write", a->logmsg_path);
-			free(msg);
+	} else if (a->merged_log) {
+		err = read_prepared_logmsg(&merged_msg, a->merged_log);
+		if (err)
 			goto done;
-		}
-		free(msg);
 	}
 
 	initial_content_len = asprintf(&initial_content,
-	    "\n# changes to be committed on branch %s:\n",
-	    a->branch_name);
+	    "%s%s\n# changes to be committed on branch %s:\n",
+	    prepared_msg ? prepared_msg : "",
+	    merged_msg ? merged_msg : "", a->branch_name);
 	if (initial_content_len == -1) {
 		err = got_error_from_errno("asprintf");
 		goto done;
@@ -8857,6 +8857,8 @@ done:
 done:
 	free(initial_content);
 	free(template);
+	free(prepared_msg);
+	free(merged_msg);
 
 	if (fd != -1 && close(fd) == -1 && err == NULL)
 		err = got_error_from_errno2("close", a->logmsg_path);
@@ -9016,7 +9018,7 @@ cmd_commit(int argc, char *argv[])
 	char *cwd = NULL, *id_str = NULL;
 	struct got_object_id *id = NULL;
 	const char *logmsg = NULL;
-	char *prepared_logmsg = NULL;
+	char *prepared_logmsg = NULL, *merged_logmsg = NULL;
 	struct collect_commit_logmsg_arg cl_arg;
 	const char *author = NULL;
 	char *gitconfig_path = NULL, *editor = NULL, *committer = NULL;
@@ -9151,9 +9153,8 @@ cmd_commit(int argc, char *argv[])
 		goto done;
 
 	if (prepared_logmsg == NULL) {
-		error = lookup_logmsg_ref(&prepared_logmsg,
-		    argc > 0 ? &paths : NULL, &refs,
-		    worktree, repo);
+		error = lookup_logmsg_ref(&merged_logmsg,
+		    argc > 0 ? &paths : NULL, &refs, worktree, repo);
 		if (error)
 			goto done;
 	}
@@ -9161,6 +9162,7 @@ cmd_commit(int argc, char *argv[])
 	cl_arg.editor = editor;
 	cl_arg.cmdline_log = logmsg;
 	cl_arg.prepared_log = prepared_logmsg;
+	cl_arg.merged_log = merged_logmsg;
 	cl_arg.non_interactive = non_interactive;
 	cl_arg.worktree_path = got_worktree_get_root_path(worktree);
 	cl_arg.branch_name = got_worktree_get_head_ref_name(worktree);