t7450: test verify_path() handling of gitmodules

t7450: test verify_path() handling of gitmodules

Commit 10ecfa7649 (verify_path: disallow symlinks in .gitmodules,
2018-05-04) made it impossible to load a symlink .gitmodules file into
the index. However, there are no tests of this behavior. Let's make sure
this case is covered. We can easily reuse the test setup created by
the matching b7b1fca175 (fsck: complain when .gitmodules is a symlink,
2018-05-04).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>