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>

environment.c

@@ -68,6 +68,11 @@ unsigned long pack_size_limit_cfg;
 #endif
 int protect_hfs = PROTECT_HFS_DEFAULT;
 
+#ifndef PROTECT_NTFS_DEFAULT
+#define PROTECT_NTFS_DEFAULT 0
+#endif
+int protect_ntfs = PROTECT_NTFS_DEFAULT;
+
 /*
  * The character that begins a commented line in user-editable file
  * that is subject to stripspace.