Error trying to write ref with nonexistent object

From @richardpon:

Occasionally the line slow query php job has clusterrunner failures.
Both have similar errors from the console:

```
ClusterRunner encountered an error processing build 9206:
'Could not update local ref. Command: "git update-ref refs/clusterrunner/86a97b5ba3e40563ed7acba73922846723ad65d0 86a97b5ba3e40563ed7acba73922846723ad65d0"
Output: "error: Trying to write ref refs/clusterrunner/86a97b5ba3e40563ed7acba73922846723ad65d0 with nonexistant object 86a97b5ba3e40563ed7acba73922846723ad65d0
fatal: Cannot update the ref 'refs/clusterrunner/86a97b5ba3e40563ed7acba73922846723ad65d0'.
"'
```