environment: make `get_git_dir()` accept a repository

environment: make `get_git_dir()` accept a repository

The `get_git_dir()` function retrieves the path to the Git directory for
`the_repository`. Make it accept a `struct repository` such that it can
work on arbitrary repositories and make it part of the repository
subsystem. This reduces our reliance on `the_repository` and clarifies
scope.

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