git-merge: fail correctly when we cannot fast forward.

When we cannot fast forward the working tree and the current
branch, git-merge did not exit with non-zero status.

Noticed by Larry Streepy, the section to be fixed identfied by
Johannes Schindelin.

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

git-merge.sh

@@ -294,7 +294,7 @@ f,*)
 	git-update-index --refresh 2>/dev/null
 	new_head=$(git-rev-parse --verify "$1^0") &&
 	git-read-tree -v -m -u --exclude-per-directory=.gitignore $head "$new_head" &&
-	finish "$new_head" "Fast forward"
+	finish "$new_head" "Fast forward" || exit
 	dropsave
 	exit 0
 	;;