Feature Request: github connector should index source code

[ethanmdavidson]

Previously discussed in #1318, but it went stale. I think this feature is a big deal, because it's hard to provide good answers without seeing the code! Documentation is constantly out of date; source code is the only thing that is guaranteed to be relevant. I am trying to use onyx to understand a large legacy code base, but it can't provide any good answers because all it sees are PRs. This is especially bad in my case because the majority of the code was written before the organization moved to github.

I think it is critical for onyx to be able to index a git repo. Source code and commit messages are the most information-dense documents available in most organizations.