It is possible to specify submodule URLs relative to the repository
location. E.g. having a submodule with URL "../submodule" will look for
the submodule at "repo/../submodule".

With the introduction of worktrees, though, we cannot simply resolve the
URL relative to the repository location itself. If the repository for
which a URL is to be resolved is a working tree, we have to resolve the
URL relative to the parent's repository path. Otherwise, the URL would
change depending on where the working tree is located.

Fix this by special-casing when we have a working tree while getting the
URL base.