git: Allow setup of reference-transaction hook with repo interface

In order to set up the reference-trannsaction hook, one needs to pass in a gitalypb.Repository. This is needlessly restrictive: the hook doesn't use any information which wouldn't be available via the repository.GitRepo interface. As a result, callers which do not have the proto available simply use the helper.ProtoRepoFromRepo() function to convert the interface to a proto.

This function is kind of dangerous though if used in different contexts: the resulting proto is lacking project-specific information like the project path and project name. So if one for example has the idea to execute a hook different than the reference-transaction hook with a proto constructed from that function, then we would probably start failing in unintelligible ways later down the road.

Let's fix the interface by instead accepting a repository.GitRepo as argument to WithRefTxHook(), constructing the protobuf right at the callsite. This ensures we're always passing a consistently-populated protobuf down to the hook, and thus there is no way that we accidentally start using info in it which may sometimes be missing.