Sync with v1.8.5.6

* maint-1.8.5: Git 1.8.5.6 fsck: complain about NTFS ".git" aliases in trees read-cache: optionally disallow NTFS .git variants path: add is_ntfs_dotgit() helper fsck: complain about HFS+ ".git" aliases in trees read-cache: optionally disallow HFS+ .git variants utf8: add is_hfs_dotgit() helper fsck: notice .git case-insensitively t1450: refactor ".", "..", and ".git" fsck tests verify_dotfile(): reject .git case-insensitively read-tree: add tests for confusing paths like ".." and ".git" unpack-trees: propagate errors adding entries to the index