Support "git rebase"

[kevinburkeomg]

I don't see a tracking issue for this, and it's not clear from the docs that it's not supported, so I thought it'd be helpful to add an issue to make clear that it's not supported, as well as to track progress (if any) toward supporting it!

[stmcginnis]

FWIW, it is marked as not supported: https://github.com/src-d/go-git/blob/master/COMPATIBILITY.md

I agree, this would be a nice one to add (along with things like merge).

[XVilka]

Really important feature for a Git library 👍