refs/files: drop `USE_THE_REPOSITORY_VARIABLE`

refs/files: drop `USE_THE_REPOSITORY_VARIABLE`

We have a bunch of users of `the_repository` in the "files" backend, all
of which are trivial to convert to instead use the backend's own repo.
Do so.

There is one more dependency on global state though via `ignore_case`,
and thus we can't trivially remove `USE_THE_REPOSITORY_VARIABLE`. But
this is the only use of global state, and we want to ensure that we
don't unwittingly reintroduce a dependency on `the_repository` going
forward.

Add an extern declaration for `ignore_case` so that it becomes
accessible even without `USE_THE_REPOSITORY_VARIABLE` and drop the
define itself.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>