current_git: support private repositories

[patricoferris]

This issue has been brought up a few times (ocurrent/ocluster#179, ocurrent/ocaml-ci#323) but I thought I would open it on the repository where the fix/feature would have to start!

Currently, current_git does not support checking out private repositories. For now, I'll limit the discussion to Github repos. There's a few ways off of the top of my head you could go about trying to fix this:

1. Passing in a Github token to be placed in the URL when doing the fetch. Something like this code with some overriding of the pp_cmd to not leak the token in the logs. This works until your submodules are also private.
2. Setup either the main account on the machine you are running to have access to Github or setup some user on the machine with access and run git as them. This doesn't feel great and probably will have permission errors.
3. There might be some sort of thing we can with git-credentials and a token to overcome the submodule problem, I'm not sure.

Any other ideas?

[tmcgilchrist]

POC by @maiste #420