This tests primarily for NTFS issues, but also adds one example of an
HFS+ issue.

Thanks go to Congyi Wu for coming up with the list of examples where
NTFS would possibly equate the filename with `.gitmodules`.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJeff King <peff@peff.net>