(this needs the small refactoring i just sent too)

as per title, add a flag to `got patch' to reverse a patch before
applying it.  I went with -R for consistency with patch(1).

With the fixed got-read-patch, reverting a diff is dead easy: just swap
the (from, lines) counters and swap every + with - before applying it.

There's a small overlap in functionalities between `got revert' and `got
patch -R' but I think it's useful to keep it.

ok?

-----------------------------------------------
commit 86e1ad73c6d5632b4f705a0ffc798e19d986795b (main)
from: Omar Polo <op@omarpolo.com>
date: Tue Apr 12 10:38:42 2022 UTC
 
 got_patch: allow to reverse a patch
 
 Allow to reverse a patch before applying it, and add the -R flag to `got
 patch'.  While here, add the forgotten `-p strip-count' to usage_patch
 too.
 
diff 0e6f5b49e77b526f4d1cfcada5b3e4a728dbe44f ba7d371282376d7ef7932c7ff6aeea2a74fcc671
blob - bd6e06c564ce9499b313b1d667097c58c2d16873
blob + b91cfb2cb4963721a9a86be9c2cdb397bc1a9278
--- got/got.1
+++ got/got.1
@@ -1285,7 +1285,7 @@ option)
 .El
 .El
 .Tg pa
-.It Cm patch Oo Fl n Oc Oo Fl p Ar strip-count Oc Op Ar patchfile
+.It Cm patch Oo Fl n Oc Oo Fl p Ar strip-count Oc Oo Fl R Oc Op Ar patchfile
 .Dl Pq alias: Cm pa
 Apply changes from
 .Ar patchfile
@@ -1367,6 +1367,8 @@ and
 path prefixes generated by
 .Xr git-diff 1
 will be recognized and stripped automatically.
+.It Fl R
+Reverse the patch before applying it.
 .El
 .Tg rv
 .It Cm revert Oo Fl p Oc Oo Fl F Ar response-script Oc Oo Fl R Oc Ar path ...
blob - 634d14fbd0d903b762d975a43ede19d0b14de466
blob + 5eac6b2ce376db8d223ef8a91c9104c7a02725d0
--- got/got.c
+++ got/got.c
@@ -7152,8 +7152,8 @@ done:
 __dead static void
 usage_patch(void)
 {
-	fprintf(stderr, "usage: %s patch [-n] [patchfile]\n",
-	    getprogname());
+	fprintf(stderr, "usage: %s patch [-n] [-p strip-count] "
+	    "[-R] [patchfile]\n", getprogname());
 	exit(1);
 }
 
@@ -7242,10 +7242,10 @@ cmd_patch(int argc, char *argv[])
 	struct got_repository *repo = NULL;
 	const char *errstr;
 	char *cwd = NULL;
-	int ch, nop = 0, strip = -1;
+	int ch, nop = 0, strip = -1, reverse = 0;
 	int patchfd;
 
-	while ((ch = getopt(argc, argv, "np:")) != -1) {
+	while ((ch = getopt(argc, argv, "np:R")) != -1) {
 		switch (ch) {
 		case 'n':
 			nop = 1;
@@ -7256,6 +7256,9 @@ cmd_patch(int argc, char *argv[])
 				errx(1, "pathname strip count is %s: %s",
 				     errstr, optarg);
 			break;
+		case 'R':
+			reverse = 1;
+			break;
 		default:
 			usage_patch();
 			/* NOTREACHED */
@@ -7303,7 +7306,7 @@ cmd_patch(int argc, char *argv[])
 		err(1, "pledge");
 #endif
 
-	error = got_patch(patchfd, worktree, repo, nop, strip,
+	error = got_patch(patchfd, worktree, repo, nop, strip, reverse,
 	    &patch_progress, NULL, check_cancelled, NULL);
 
 done:
blob - 62c76b0eb2926a03364ef10e416183cdfa18f398
blob + bc2e95bb4fa535617bc28f76935a78e36dea8a3e
--- include/got_patch.h
+++ include/got_patch.h
@@ -32,4 +32,4 @@ typedef const struct got_error *(*got_patch_progress_c
  */
 const struct got_error *
 got_patch(int, struct got_worktree *, struct got_repository *, int, int,
-    got_patch_progress_cb, void *, got_cancel_cb, void *);
+    int, got_patch_progress_cb, void *, got_cancel_cb, void *);
blob - fd8a331bcf22f2eae5872f93fe383f4de9dd7af6
blob + 556b5fc43838fc9d635d2795c68d11b20d0391ef
--- lib/patch.c
+++ lib/patch.c
@@ -674,10 +674,39 @@ done:
 	return err;
 }
 
+static void
+reverse_patch(struct got_patch *p)
+{
+	struct got_patch_hunk *h;
+	size_t i;
+	long tmp;
+
+	STAILQ_FOREACH(h, &p->head, entries) {
+		tmp = h->old_from;
+		h->old_from = h->new_from;
+		h->new_from = tmp;
+
+		tmp = h->old_lines;
+		h->old_lines = h->new_lines;
+		h->new_lines = tmp;
+
+		tmp = h->old_nonl;
+		h->old_nonl = h->new_nonl;
+		h->new_nonl = tmp;
+
+		for (i = 0; i < h->len; ++i) {
+			if (*h->lines[i] == '+')
+				*h->lines[i] = '-';
+			else if (*h->lines[i] == '-')
+				*h->lines[i] = '+';
+		}
+	}
+}
+
 const struct got_error *
 got_patch(int fd, struct got_worktree *worktree, struct got_repository *repo,
-    int nop, int strip, got_patch_progress_cb progress_cb, void *progress_arg,
-    got_cancel_cb cancel_cb, void *cancel_arg)
+    int nop, int strip, int reverse, got_patch_progress_cb progress_cb,
+    void *progress_arg, got_cancel_cb cancel_cb, void *cancel_arg)
 {
 	const struct got_error *err = NULL;
 	struct got_fileindex *fileindex = NULL;
@@ -736,6 +765,9 @@ got_patch(int fd, struct got_worktree *worktree, struc
 		if (err || done)
 			break;
 
+		if (reverse)
+			reverse_patch(&p);
+
 		err = got_worktree_patch_check_path(p.old, p.new, &oldpath,
 		    &newpath, worktree, repo, fileindex);
 		if (err == NULL)
blob - 24c65df8f37d93d010464929c0370750824f4534
blob + f8a053a6f93237b3aa62a9f0e0f3df06d4922010
--- regress/cmdline/patch.sh
+++ regress/cmdline/patch.sh
@@ -1273,6 +1273,50 @@ EOF
 	test_done $testroot 0
 }
 
+test_patch_reverse() {
+	local testroot=`test_init patch_reverse`
+
+	got checkout $testroot/repo $testroot/wt > /dev/null
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		test_done $testroot $ret
+		return 1
+	fi
+
+	cat <<EOF > $testroot/wt/patch
+--- alpha
++++ alpha
+@@ -1 +1 @@
+-ALPHA
+\ No newline at end of file
++alpha
+EOF
+
+	(cd $testroot/wt && got patch -R patch) > $testroot/stdout
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		test_done $testroot $ret
+		return 1
+	fi
+
+	echo "M  alpha" > $testroot/stdout.expected
+	cmp -s $testroot/stdout.expected $testroot/stdout
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		diff -u $testroot/stdout.expected $testroot/stdout
+		test_done $testroot $ret
+		return 1
+	fi
+
+	echo -n ALPHA > $testroot/wt/alpha.expected
+	cmp -s $testroot/wt/alpha.expected $testroot/wt/alpha
+	ret=$?
+	if [ $ret -ne 0 ]; then
+		diff -u $testroot/wt/alpha.expected $testroot/wt/alpha
+	fi
+	test_done $testroot $ret
+}
+
 test_parseargs "$@"
 run_test test_patch_simple_add_file
 run_test test_patch_simple_rm_file
@@ -1294,3 +1338,4 @@ run_test test_patch_with_offset
 run_test test_patch_prefer_new_path
 run_test test_patch_no_newline
 run_test test_patch_strip
+run_test test_patch_reverse