setup: split up concerns of `setup_git_env_internal()`

setup: split up concerns of `setup_git_env_internal()`

The function `setup_git_env_internal()` does two completely unrelated
things:

  - It configures the repository's gitdir and propagates environment
    variables into it.

  - It configures a couple of global parameters via environment
    variables.

The function is called when we initialize the repository's path, but
it's also called via `chdir_notify_register()` whenever we change the
current working directory. While we indeed have to reconfigure the
gitdir in case it's a relative path, it doesn't make sense to reapply
the global environment variables.

Split up concerns of this function along the above delineation. Handling
of the global environment variables is moved into `init_git()`, as they
can be considered part of our setup procedure.

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