Support git credential manager

[brettswift]

First off, love this cli tool.

I find when using it (Version 0.5.1) when I'm configured to use the Git Credential Manager (https://github.com/git-ecosystem/git-credential-manager) it wants to pop up an auth window for every repository.

It would be nice if it didn't do this :)

Thanks for all the work here!

[earwig]

Thanks for reporting the issue – I'm guessing it must be something in GitPython since we rely on that for all of our interactions with git, but will try to reproduce.