Add git subdirectory tests about mentat

[PCSwingle]

Sorry I wasn't clear! I'm pretty sure it works as expected right now (I could be wrong); if you run it from within a subdirectory of a git repo, it correctly picks up the git root, and when you specify files when starting mentat it correctly picks the files relative to your cwd; all this issue is for is adding tests to make sure these things work correctly when the cwd is a subdirectory of the git repo. Hopefully that makes more sense, let me know if you have any other questions!

[granawkins]

I can do this quick but just to clarify, what's the expected behavior exactly?

e.g. in Mentat, I navigate to mentat/mentat, then run mentat code_file.py, and it runs with the top-level mentat as the git root. (this needs one fix to work, code_file_manager.py line 50)

Are you meant to also specify paths in sibling directories? e.g. mentat code_change.py ../tests/code_change_test.py?

[granawkins]

Resolved by #96