remote-hg: fix new branch creation

remote-hg: fix new branch creation

When a user creates a new branch with git:

 % git checkout -b branches/devel

and then pushes this branch

 % git push origin branches/devel

which is the way to push new mercurial branches, we do want to
create a branch, but the command would fail without newbranch=True.

This only matters when force_push=False, but setting newbranch=True
unconditionally does not hurt.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>