Fix rev-graph-update

What occurred:

1. target-helper had an error update and rejected a job.
2. jobs deleted the job from the pending queue
3. a rev-graph update came in late
4. then the final job fail status came in

As a result a job entry only had a _rev key. We just want to make sure that final change is not lost entirely by the job watcher.

[
  {
    "resource_id": "resources/29xL9znRy5BfCNxdoOfo3pkxJ4N",
    "path": "",
    "body": {
      "2A1k6RS0YOuzlD4wtDiNTUZe3E8": null,
      "_meta": {
        "modifiedBy": "users/433",
        "modified": 1654267058.506,
        "_rev": 4087
      },
      "_rev": 4087
    },
    "type": "delete"
  }
]

[
  {
    "resource_id": "resources/29xL9znRy5BfCNxdoOfo3pkxJ4N",
    "path": "",
    "body": {
      "2A1k6RS0YOuzlD4wtDiNTUZe3E8": {
        "_rev": 8
      },
      "_meta": {
        "modifiedBy": "system/rev_graph_update",
        "modified": 1654267058.59,
        "_rev": 4088
      },
      "_rev": 4088
    },
    "type": "merge"
  },
  {
    "resource_id": "resources/2A1k6RS0YOuzlD4wtDiNTUZe3E8",
    "path": "/2A1k6RS0YOuzlD4wtDiNTUZe3E8",
    "body": {
      "status": "failure",
      "result": {
        "name": "TimeoutError"
      },
      "_meta": {
        "modifiedBy": "users/433",
        "modified": 1654267053.628,
        "_rev": 8
      },
      "_rev": 8
    },
    "type": "merge"
  }
]

No files or tests are named. Start by locating the job watcher and rev-graph update handling, then trace the event sequence shown in the report. Done means the late rev-graph update and final failure status are not lost, and the job entry does not end up containing only a _rev key.