subtree: allow 'split' flags to be passed to 'push'

subtree: allow 'split' flags to be passed to 'push'

'push' does a 'split' internally, but it doesn't pass flags through to the
'split'.  This is silly, if you need to pass flags to 'split', then it
means that you can't use 'push'!

So, have 'push' accept 'split' flags, and pass them through to 'split'.

Add tests for this by copying split's tests with minimal modification.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>