push: add shorthand for --force-with-lease branch creation

push: add shorthand for --force-with-lease branch creation

Allow the empty string to stand in for the null SHA-1 when pushing a new
branch, like we do when deleting branches.

This means that the following command ensures that `new-branch` is
created on the remote (that is, is must not already exist):

	git push --force-with-lease=new-branch: origin new-branch

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>