read-cache: optionally disallow NTFS .git variants

The point of disallowing ".git" in the index is that we would never want to accidentally overwrite files in the repository directory. But this means we need to respect the filesystem's idea of when two paths are equal. The prior commit added a helper to make such a comparison for NTFS and FAT32; let's use it in verify_path(). We make this check optional for two reasons: 1. It restricts the set of allowable filenames, which is unnecessary for people who are not on NTFS nor FAT32. In practice this probably doesn't matter, though, as the restricted names are rather obscure and almost certainly would never come up in practice. 2. It has a minor performance penalty for every path we insert into the index. This patch ties the check to the core.protectNTFS config option. Though this is expected to be most useful on Windows, we allow it to be set everywhere, as NTFS may be mounted on other platforms. The variable does default to on for Windows, though. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>