don't check for changed files when checking out a new branch

[ianhi]

Checking out a new branch will never cause files to change so we don't need to to check changed files. The attempt to check was failing because we were asking for the git diff between the current branch and one that did not yet exist

Fixes: #828

[github-actions]

👈 Launch a binder notebook on branch ianhi/jupyterlab-git/issue828

[fcollonval]

@ianhi Thanks a lot for the quick fix.

Would you have time to add unit tests on this feature and the one addressed by #824 to reduce potential troubles in the future?

it would make sense to put them in https://github.com/jupyterlab/jupyterlab-git/blob/master/tests/model.spec.tsx#L299. I think you could assert internal calls using spy like https://github.com/jupyterlab/jupyterlab-git/blob/master/tests/model.spec.tsx#L365.

[ianhi]

Will do. Thanks for the pointers on how to do it, makes it much more approachable.

[fcollonval]

LGTM

I merge this one to cut a patch release. But if you have time to add some tests in another PR, that would be much appreciate.