merge-recursive: avoid triggering add_cacheinfo error with dirty mod

If a cherry-pick or merge with a rename results in a skippable update
(due to the merged content matching what HEAD already had), but the
working directory is dirty, avoid trying to refresh the index as that
will fail.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -2786,7 +2786,7 @@ static int merge_content(struct merge_options *o,
 		path_renamed_outside_HEAD = !path2 || !strcmp(path, path2);
 		if (!path_renamed_outside_HEAD) {
 			if (add_cacheinfo(o, mfi.mode, &mfi.oid, path,
-					  0, (!o->call_depth), 0))
+					  0, (!o->call_depth && !is_dirty), 0))
 				return -1;
 			return mfi.clean;
 		}