nip34: add refs to git repo event

[DanConwayDev]

so that the event can be used as a source of truth for the state of refs such as branches and tags.

this could be useful as:

1. a way to reduce trust in git server.(s) so they no longer act as a source of truth. A nip34 git remote-helper could proxy requests to git servers and only pull updates when they match the state listed in the repo event.
2. a form of authorisation for nip34 git server implementations such as song
3. a way of enabling experimentation with other protocols for hosting and accessing git data. eg a blossom git remote helper