contrib/subtree: Teach push to abort if split fails

If the subtree split fails it doesn't return the SHA that should be
pushed to the other repository.

Added a check to ensure that split succeeds before trying to push
and display an error message.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>
contrib/subtree: Teach push to use --force option

Allow the --force option to be passed to the child push command.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>