fetch: exit non-zero when fast-forward check fails.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-fetch.sh

@@ -179,6 +179,7 @@ fast_forward_local () {
 			;;
 		*)
 			echo >&2 "  not updating."
+			exit 1
 			;;
 		esac
 	    }