git kill branchname

@charlierudolph Aaaaahhhh, for some reason some commits meant for this PR ended up in an already merged PR. I was messing around with `git extract` while being distracted on the resource planning call, so that's my (super lame) excuse... Anyways, please take a look at https://github.com/Originate/git-town/commit/738e3e76158a4803c5695abf71349b21d02033f5, and comment there. I will implement the corresponding fixes in this pull request, and once this gets merged its all going to be kind of okay again...  :smile: 
There is an unnecessary checkout when undoing a kill by branch name.
I would think the logic is more like this:

```bash
if [ "$branch_to_kill" == "$initial_branch_name" ]; then
  add_to_undo_script "create_and_checkout_branch $branch_to_kill $current_sha"
  if [ "$initial_open_changes" == true ]; then
    add_to_undo_script "reset_to_sha $original_branch_sha"
  fi
  if [ "$(has_tracking_branch "$initial_branch_name")" == true ]; then
    add_to_undo_script "push_branch"
  fi
else
  add_to_undo_script "create_branch $branch_to_kill $current_sha"
fi
```