gc --auto --quiet: make the notice a bit less verboase

When "gc --auto --quiet" decides there is something to do, it tells the
user what it is doing, as it is going to make the user wait for a bit.

But the message was a bit too long.

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

builtin-gc.c

@@ -216,10 +216,13 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		 */
 		if (!need_to_gc())
 			return 0;
-		fprintf(stderr, "Auto packing your repository for optimum "
-			"performance. You may also\n"
-			"run \"git gc\" manually. See "
-			"\"git help gc\" for more information.\n");
+		fprintf(stderr,
+			"Auto packing the repository for optimum performance.%s\n",
+			quiet
+			? ""
+			: (" You may also\n"
+			   "run \"git gc\" manually. See "
+			   "\"git help gc\" for more information."));
 	} else
 		append_option(argv_repack,
 			      prune_expire && !strcmp(prune_expire, "now")