Merge branch 'jk/rerere-forget-check-enabled' into maint

"git rerere forget" in a repository without rerere enabled gave a
cryptic error message; it should be a silent no-op instead.

* jk/rerere-forget-check-enabled:
  rerere: exit silently on "forget" when rerere is disabled

rerere.c

@@ -659,6 +659,8 @@ int rerere_forget(struct pathspec *pathspec)
 		return error("Could not read index");
 
 	fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
+	if (fd < 0)
+		return 0;
 
 	unmerge_cache(pathspec);
 	find_conflict(&conflict);