Support tags and/or revisions in addition to branches

[agoose77]

Right now, we can only provide a branch name to nbgitpuller. In the context of mystmd, I'd like to re-use the same context used by the Binder integration, namely ref.

I know there's been discussion about HEAD, but has revisions been spoken about before?

[yuvipanda]

tags and revisions should probably already work - what issue do you run into?