contrib/subtree: respect spaces in a repository path

Remote repository may have spaces in its path, so take it into account.

Also, as far as there are no tests for the `push` command, add them.

Signed-off-by: Alexey Shumkin <Alex.Crezoff@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -735,7 +735,7 @@ cmd_push()
 	    refspec=$2
 	    echo "git push using: " $repository $refspec
 	    localrev=$(git subtree split --prefix="$prefix") || die
-	    git push $repository $localrev:refs/heads/$refspec
+	    git push "$repository" $localrev:refs/heads/$refspec
 	else
 	    die "'$dir' must already exist. Try 'git subtree add'."
 	fi