patch: reapply path limit

This was originally applied added in commit eac1b67 (ui-patch: Apply
path limit to generated patch, 2010-06-10) but the ability to limit
patches to particular paths was lost in commit 455b598 (ui-patch.c: Use
log_tree_commit() to generate diffs, 2013-08-20).

The new output is slightly different from the original because Git's
diff infrastructure doesn't give us a way to insert an annotation
immediately after the "---" separator, so the commit has moved below the
diff stat.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-patch.c

@@ -18,9 +18,13 @@ void cgit_print_patch(const char *new_rev, const char *old_rev,
 	struct commit *commit;
 	struct object_id new_rev_oid, old_rev_oid;
 	char rev_range[2 * 40 + 3];
-	char *rev_argv[] = { NULL, "--reverse", "--format=email", rev_range };
+	const char *rev_argv[] = { NULL, "--reverse", "--format=email", rev_range, "--", prefix };
+	int rev_argc = ARRAY_SIZE(rev_argv);
 	char *patchname;
 
+	if (!prefix)
+		rev_argc--;
+
 	if (!new_rev)
 		new_rev = ctx.qry.head;
 
@@ -79,7 +83,9 @@ void cgit_print_patch(const char *new_rev, const char *old_rev,
 	rev.max_parents = 1;
 	rev.diffopt.output_format |= DIFF_FORMAT_DIFFSTAT |
 			DIFF_FORMAT_PATCH | DIFF_FORMAT_SUMMARY;
-	setup_revisions(ARRAY_SIZE(rev_argv), (const char **)rev_argv, &rev,
+	if (prefix)
+		rev.diffopt.stat_sep = fmt("(limited to '%s')\n\n", prefix);
+	setup_revisions(ARRAY_SIZE(rev_argv), rev_argv, &rev,
 			NULL);
 	prepare_revision_walk(&rev);