[PATCHv2] setup and GIT_IMPLICIT_WORK_TREE

Here's a re-roll of the GIT_IMPLICIT_WORK_TREE patch which should address the comments in the last round. I've added an explanatory comment near the variable definition, and added it to local_repo_env.

While doing that, I noticed some cleanup opportunities around local_repo_env, which resulted in the first two patches.

  [1/3]: cache.h: drop LOCAL_REPO_ENV_SIZE
  [2/3]: environment: add GIT_PREFIX to local_repo_env
  [3/3]: setup: suppress implicit "." work-tree for bare repos

-Peff