subtree: push: allow specifying a local rev other than HEAD

subtree: push: allow specifying a local rev other than HEAD

'git subtree split' lets you specify a rev other than HEAD.  'git push'
lets you specify a mapping between a local thing and a remot ref.  So
smash those together, and have 'git subtree push' let you specify which
local thing to run split on and push the result of that split to the
remote ref.

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