environment: make `get_git_common_dir()` accept a repository

environment: make `get_git_common_dir()` accept a repository

The `get_git_common_dir()` function retrieves the path to the common
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>