Simplify git-kill branch name

@kevgo 

* Only run `ensure_has_branch` if killing by name
* No need to stash/restore open changes as will never leave current branch if killing by name
* Have `commit_open_changes` check for `$initial_open_changes`
  * tried to mention this a few times but never explained it well enough
We often use `== true`. Is there a reason why you use only one `=` here?