Fix target ref logging in merge commit error message

Cast the protobuf byte string to a string, otherwise the target_ref value appears as a base64-encoded message.

Tested locally by hacking the OID to use 123456:

{
  "correlation_id": "01F9CXFS28H0NX6N9Z9FHP0T6J",
  "error": "merge: ours commit lookup: lookup commit \"123456\": revspec '123456' not found\n",
  "grpc.meta.auth_version": "v2",
  "grpc.meta.client_name": "gitlab-web",
  "grpc.meta.deadline_type": "regular",
  "grpc.method": "UserMergeToRef",
  "grpc.request.deadline": "2021-06-29T22:16:54.073Z",
  "grpc.request.fullMethod": "/gitaly.OperationService/UserMergeToRef",
  "grpc.request.glProjectPath": "root/test",
  "grpc.request.glRepository": "project-41",
  "grpc.request.repoPath": "@hashed/3d/91/3d914f9348c9cc0ff8a79716700b9fcd4d2f3e711608004eb8f138bcba7f14d9.git",
  "grpc.request.repoStorage": "default",
  "grpc.service": "gitaly.OperationService",
  "grpc.start_time": "2021-06-29T22:15:59.173Z",
  "level": "error",
  "msg": "unable to create merge commit",
  "peer.address": "@",
  "pid": 20050,
  "remote_ip": "X.X.X.X",
  "source_sha": "7ea5effa2885dd284244ee0cfcd4c58cc01daf67",
  "span.kind": "server",
  "system": "grpc",
  "target_ref": "refs/merge-requests/3/merge",
  "target_sha": "2954282a035331ceec532dd8be5f044863e4f8e4",
  "time": "2021-06-29T22:15:59.330Z",
  "username": "root"
}