Support nested Git repositories

[zxqfl]

root
|
|-- subproj1
|
|-- subproj2
    |
    |-- subsubproj1
    |
    |-- subsubproj2

Currently TabNine will stop at the first .git it finds, but it should continue to try to find a .tabnine_root.

[trishume]

Watch out, some people have their home directory as a git directory for dotfiles purposes. I think this is a bad idea but in nevertheless happens and might be worth a special case to avoid.

[zxqfl]

This should be fine -- we still stop at the first .git, we only keep looking for a .tabnine_root. (So users with nested Git repos need to put a tabnine_root in the parent repo.)