environment: reorder header to split out `the_repository`-free section

environment: reorder header to split out `the_repository`-free section

Reorder the "environment.h" header such that declarations which are free
from `the_repository` come before those which aren't. The new structure
is now:

    - Defines for environment variable names.

    - Things which do not rely on a repository.

    - Things which do, including those that implicitly rely on a parsed
      repository. This includes for example variables which get
      populated when reading repository config.

This will allow us to guard the last category of declarations with
`USE_THE_REPOSITORY_VARIABLE`.

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