When updating references in UserMergeBranch failed due to a concurrent
update of the same reference, then Gitaly used to return no error at all
but instead just return an empty gRPC response. This behaviour isn't
obvious at all and easy to get wrong. Furthermore, Rails didn't have any
information about what went wrong.

To fix this, Gitaly has introduced a new error type which is returned in
case there was a race updating the reference. Instead of returning
successful, it now returns an error with a ReferenceUpdateError embedded
inside of it. This allow us to extract this error and thus know exactly
what happened, including the reference that failed to update with both
its old and new expected object ID values.

Update the Gitaly client to handle this error in a backwards-compatible
fashion. For now this means that we silently ignore it like we used to
do with the old error as well.