branch: make create_branch() always create a branch

branch: make create_branch() always create a branch

With the previous commit, there are no more invocations of
create_branch() that do not create a branch because:

* BRANCH_TRACK_OVERRIDE is no longer passed
* clobber_head_ok = true and force = false is never passed

Assert these situations, delete dead code and ensure that we're handling
clobber_head_ok and force correctly by introducing tests for `git branch
--force`. As a result, create_branch() now always creates a branch.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>