Avoid initializing nested Git repository

Previously a Git repository was initialized if a Cargo workspace was
detected. However, it's also possible for users to initialize rustlings
in an existing Git repository that doesn't contain a Cargo workspace. In
that case, it's still undesirable to initialize a nested Git repository
for rustlings. We therefore search all ancestors of the current working
directory for `.git` or `.jj` directories to determine if rustlings is
being initialized in an existing Git repository.