refs.c::do_for_each_ref(): Finish error message lines with "\n"

We used fprintf() to show an error message without terminating
it with LF; use error() for that.

cf. c401cb48Signed-off-by: NJunio C Hamano <junkio@cox.net>

refs.c

@@ -152,12 +152,12 @@ static int do_for_each_ref(const char *base, int (*fn)(const char *path, const u
 				continue;
 			}
 			if (read_ref(git_path("%s", path), sha1) < 0) {
-				fprintf(stderr, "%s points nowhere!", path);
+				error("%s points nowhere!", path);
 				continue;
 			}
 			if (!has_sha1_file(sha1)) {
-				fprintf(stderr, "%s does not point to a valid "
-						"commit object!", path);
+				error("%s does not point to a valid "
+				      "commit object!", path);
 				continue;
 			}
 			retval = fn(path, sha1);