Needed to perform this request was only the helper already implemented,
however this one method RepoPath(), first checked the storage,
constructed a path, guarded against path traversal, and checked if the
path passed was a git repo. My implementation tries to split these concern
without changing the API for now.

The request can only respond with an `InvalidArgument` error code. If it
returns an error, the `Exists` property is not set. In other
cases it checks if the directory has the `objects` folder and `HEAD`
file.