Code clean-up for the "repository" abstraction.

* nd/remove-ignore-env-field:
  repository.h: add comment and clarify repo_set_gitdir
  repository: delete ignore_env member
  sha1_file.c: move delayed getenv(altdb) back to setup_git_env()
  repository.c: delete dead functions
  repository.c: move env-related setup code back to environment.c
  repository: initialize the_repository in main()