Improve git check-attr

This might be a way to fix &9006 and gitaly#4499 (closed), and remove our ApplyGitattributes gRPC. This might be an alternative to: #119 (closed)

This would be about making git check-attr work on a bare repo, so that Gitaly could use it, and then perhaps making it accept a revision, so that it could work with any revision of the .gitattributes file(s).

/cc @jcaigitlab @toon