fast-import: disallow more path components

fast-import: disallow more path components

Instead of just disallowing '.' and '..', make use of verify_path() to
ensure that fast-import will disallow anything we wouldn't allow into
the index, such as anything under .git/, .gitmodules as a symlink, or
a dos drive prefix on Windows.

Since a few fast-export and fast-import tests that tried to stress-test
the correct handling of quoting relied on filenames that fail
is_valid_win32_path(), such as spaces or periods at the end of filenames
or backslashes within the filename, turn off core.protectNTFS for those
tests to ensure they keep passing.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>