Update "git diff --no-index" codepath not to try to peek into .git/
directory that happens to be under the current directory, when we
know we are operating outside any repository.
* jk/no-looking-at-dotgit-outside-repo:
diff: handle sha1 abbreviations outside of repository
diff_aligned_abbrev: use "struct oid"
diff_unique_abbrev: rename to diff_aligned_abbrev
find_unique_abbrev: use 4-buffer ring
test-*-cache-tree: setup git dir
read info/{attributes,exclude} only when in repository