contrib/subtree: don't delete remote branches if split fails

When using "git subtree push" to split out a subtree and push it to a
remote repository, we do not detect if the split command fails which
causes the LHS of the refspec to be empty, deleting the remote branch.

Fix this by pulling the result of the split command into a variable so
that we can die if the command fails.

Reported-by: Steffen Jaeckel <steffen.jaeckel@stzedn.de>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -715,7 +715,8 @@ cmd_push()
 	    repository=$1
 	    refspec=$2
 	    echo "git push using: " $repository $refspec
-	    git push $repository $(git subtree split --prefix=$prefix):refs/heads/$refspec
+	    localrev=$(git subtree split --prefix="$prefix") || die
+	    git push $repository $localrev:refs/heads/$refspec
 	else
 	    die "'$dir' must already exist. Try 'git subtree add'."
 	fi