Protect against 8.3 "short name" attacks also on Linux/macOS

The Windows Subsystem for Linux (WSL) is getting increasingly popular,
in particular because it makes it _so_ easy to run Linux software on
Windows' files, via the auto-mounted Windows drives (`C:\` is mapped to
`/mnt/c/`, no need to set that up manually).

Unfortunately, files/directories on the Windows drives can be accessed
via their _short names_, if that feature is enabled (which it is on the
`C:` drive by default).

Which means that we have to safeguard even our Linux users against the
short name attacks.

Further, while the default options of CIFS/SMB-mounts seem to disallow
accessing files on network shares via their short names on Linux/macOS,
it _is_ possible to do so with the right options.

So let's just safe-guard against short name attacks _everywhere_.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

src/checkout.c

@@ -1271,7 +1271,7 @@ static int checkout_verify_paths(
 	int action,
 	git_diff_delta *delta)
 {
-	unsigned int flags = GIT_PATH_REJECT_WORKDIR_DEFAULTS;
+	unsigned int flags = GIT_PATH_REJECT_WORKDIR_DEFAULTS | GIT_PATH_REJECT_DOT_GIT_NTFS;
 
 	if (action & CHECKOUT_ACTION__REMOVE) {
 		if (!git_path_isvalid(repo, delta->old_file.path, delta->old_file.mode, flags)) {

tests/checkout/nasty.c

@@ -206,9 +206,8 @@ void test_checkout_nasty__dot_git_dot(void)
  */
 void test_checkout_nasty__git_tilde1(void)
 {
-#ifdef GIT_WIN32
 	test_checkout_fails("refs/heads/git_tilde1", ".git/foobar");
-#endif
+	test_checkout_fails("refs/heads/git_tilde1", "git~1/foobar");
 }
 
 /* A tree that contains an entry "git~2", when we have forced the short