An older cleanup of the Git path and state management broke support for

bare Git repositories. We were computing a "top-level" path for Git, and

further processing this when encountering a standard (non-bare)

repository. However, just due to a mistake, we failed to set a value for

bare repositories. This would later resut in an assertion failure.
This is a pretty simple change that sets this path in all cases. The

only case where it wouldn't set now is when we also can't compute a

.git path at all.
Nearby code has been updated to help keep type safety, ensuring we were

working off of locally-validated types for variables before buiding

paths off the .git directory, and then asserting at the end that we

had a valid top-level path.

Tested generating a diff on a bare Git repository, which previously

had failed.
Unit tests pass.