Files named PRN* fail to clone on Windows

Repositories with files matching `PRN*` fail to clone on native Windows git. They work fine in Cygwin and WSL.

E.g., `apertium-aze` fails because it contains `tests/morphotactics/prefixes/PRN.prefix.dix`

Tagging this `wontfix` right off the bat, since it's not something we should have to care about and there are workarounds for the affected platform. But it needs to be documented.