path: stop relying on `the_repository` in `worktree_git_path()`

path: stop relying on `the_repository` in `worktree_git_path()`

When not provided a worktree, then `worktree_git_path()` will fall back
to returning a path relative to the main repository. In this case, we
implicitly rely on `the_repository` to derive the path. Remove this
dependency by passing a `struct repository` as parameter.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>