R1:b0bf1c689a9d

Avoid RuntimeException passing bogus "service" parameter to Diffusion commit view

Summary:
`PhabricatorRepository::parseRepositoryServicePath()` can return `null` per https://we.phorge.it/source/phorge/browse/master/src/applications/repository/storage/PhabricatorRepository.php;123831b53fb7572cba11e9c990dcb9d247614890$635, thus make the code in `DiffusionServeController:getRequestDirectoryPath()` at least not crash when handling `null` and instead fall back to serving a "403 This repository is read-only…