builtin-fetch-pack: use warning() instead of fprintf(stderr, "warning: ")

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch-pack.c

@@ -605,7 +605,7 @@ static struct ref *do_fetch_pack(int fd[2],
 			/* When cloning, it is not unusual to have
 			 * no common commit.
 			 */
-			fprintf(stderr, "warning: no common commits\n");
+			warning("no common commits");
 
 	if (get_pack(fd, pack_lockfile))
 		die("git fetch-pack: fetch failed.");