Cannot handle empty ReferenceUpdateRequest

[scabala]

Hello,
I've been trying to implement a Git server using this library. I have encountered an issue when pushing to it. When pushing first time with some commits present in local repo that are not in remote repo, everything is fine. When I try to push again immediately afterwards, push fails. The same action works fine for GitHub, GitLab and other git services.

I have used example from here.

I have done some debugging and pinned it down to updReqDecoder's Decode method. From my investigation, it turns out that git client responds with just flush-pkt line when there are no refs to be updated - in other words, nothing to push by client. However, ReferenceUpdateRequest and its decoder, is coded in such a way that it always expects some refs being sent. That causes it to fail with capabilities delimiter not found - it comes from here.

I've looked at ReferenceUpdateRequest and it is not straightforward for me how to fix it - maybe someone else have better idea? Nonetheless, I can craft a PR if you are willing to accept it.

Thanks for all the hard work on go-git!