[PATCH 0/7] stop blind fallback to ".git"

This is a follow-on to the series in:

  http://public-inbox.org/git/20160913032242.coyuhyhn6uklewuk@sigill.intra.peff.net/

That series avoided looking at ".git/config" when we haven't discovered whether we are in a git repo. This takes that further and avoids ever looking at ".git" as a fallback.

Patches 1-6 just teach various low-level code to detect and handle the case when we are not in a configure repository (along with associated cleanups). The final patch actually disallows this case (and the early fixes were found by running the test suite with just the final patch).

I think this is a step in the right direction, both in fixing bugs, but also in making our setup and repository-access code easier to reason about. However, it does carry a risk of regression, if there are more "fixes" that I missed. If we wanted to be really conservative, we could hold back the 7th patch as a separate topic and cook it in "next" for an extra release cycle or something. I'm not all that worried, though.

I built this on top of jc/diff-unique-abbrev-comments, as it refactors that same function (and it didn't seem like a bad topic to be held hostage by).

  [1/7]: read info/{attributes,exclude} only when in repository
  [2/7]: test-*-cache-tree: setup git dir
  [3/7]: find_unique_abbrev: use 4-buffer ring
  [4/7]: diff_unique_abbrev: rename to diff_aligned_abbrev
  [5/7]: diff_aligned_abbrev: use "struct oid"
  [6/7]: diff: handle sha1 abbreviations outside of repository
  [7/7]: setup_git_env: avoid blind fall-back to ".git"

 attr.c                           |  6 +++++-
 builtin/merge.c                  | 11 +++++-----
 builtin/receive-pack.c           | 16 ++++++---------
 cache.h                          |  4 ++--
 combine-diff.c                   |  6 +++---
 diff.c                           | 43 +++++++++++++++++++++++++---------------
 diff.h                           |  6 +++++-
 dir.c                            | 12 +++++------
 environment.c                    |  5 ++++-
 sha1_name.c                      |  4 +++-
 t/helper/test-dump-cache-tree.c  |  1 +
 t/helper/test-scrap-cache-tree.c |  1 +
 12 files changed, 68 insertions(+), 47 deletions(-)