feature: add git metadata

[reteps]

Is your feature request related to a problem? Please describe.

Requirement for mkdocstrings/python#253

Describe the solution you'd like

• If a repo is loaded with load_git, parse the git metadata (i.e. the remote and ref)
• when we load a file (https://github.com/mkdocstrings/griffe/blob/main/src/_griffe/loader.py#L140 i believe), check if it is in the git repo.

I think something like git ls-files, and then checking if the file you are looking at is in that set would work great.