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

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

Stop using `the_repository` in `setup_git_env()` and instead accept the
repository as a parameter. The injection of `the_repository` is thus
bumped one level higher, where callers now pass it in explicitly.

Furthermore, the function is never used outside of "setup.c". Drop the
declaration in "environment.h" and make it static.

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