git: add references for Git pull request heads When a pull request is created, stick a reference to it in the refs/pull namespace, similarly to what GitHub does, and remove it when the pull request is deleted. That reference guarantees the commits stay around and don't get garbage-collected when a PR is rebased or revised. Also, that makes it possible to access head of historic pull requests using Git by fetching a ref from the source repository. Unlike GitHub though, we don't put the ref into the destination repository and don't copy commits there to prevent undesired repository growth. Kallithea uses global pull request identifiers, so there should not be any confusion as to what pull request the ref corresponds to. We may later provide a shim to redirect users to the source repository if that deems needed.