hooks: remove implicit dependency on `the_repository`

hooks: remove implicit dependency on `the_repository`

We implicitly depend on `the_repository` in our hook subsystem because
we use `strbuf_git_path()` to compute hook paths. Remove this dependency
by accepting a `struct repository` as parameter instead.

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