setup: stop using `the_repository` in `is_inside_work_tree()`

setup: stop using `the_repository` in `is_inside_work_tree()`

Similar as with the preceding commit, `is_inside_work_tree()` determines
whether the current working directory is located inside the worktree of
`the_repository`. Perform the same refactoring by dropping the caching
mechanism and injecting the repository that shall be checked.

Note that, same as in the preceding commit, we're also resolving the
worktree path via `realpath()`. In theory this step is not necessary as
we always set the worktree path via `repo_set_worktree()`, and that
function already resolves the path for us. But resolving the path a
second time is unlikely to matter performance-wise, and it feels fragile
to rely on the repository's worktree path being absolute. We thus
perform the same extra step even though it's ultimately not required.

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