branch: do not rename a branch under bisect or rebase

branch: do not rename a branch under bisect or rebase

The branch name in that case could be saved in rebase's head_name or
bisect's BISECT_START files. Ideally we should try to update them as
well. But it's trickier (*). Let's play safe and see if the user
complains about inconveniences before doing that.

(*) If we do it, bisect and rebase need to provide an API to rename
branches. We can't do it in worktree.c or builtin/branch.c because
when other people change rebase/bisect code, they may not be aware of
this code and accidentally break it (e.g. rename the branch file, or
refer to the branch in new files). It's a lot more work.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>