Avoid the need for a specific git checkout

[blink1073]

Problem

It is awkward to use jupyter-releaser as a CLI, for instance to iteratively check links, because it is creating a .jupyter_releaser_checkout folder. The folder also shows up in git status, unless you add a user-level git ignore.

Proposed Solution

Change the semantics to only require a git check of any kind for those commands that need one.