gbp-pull: similar update strategy for all branches

Treat non-checked-out branches similarly to the current branch when
forcing update. That is, do git merge, instead of just setting the ref.

Also, renames fast_forward_branch() to update_branch() to better match
the functionality.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Conflicts:
	gbp/scripts/pull.py