RefService: implement FindRefsByOID
This is a new RPC method defined in Gitaly protocol v14.5. It is more
than reminiscent of the FindRefNames
that upstream removed in v14.3.
On the Rails side, as of v14.5.2, FindRefsByOID
is exposed as Git::Repository#refs_by_oid
whose only caller is Git::Commit#first_ref_by_oid
, itself used
only in the Jira integration and behind a feature flag.
Therefore, priority is low, especially compared to #72 (closed)