The full error message:

```
2017-07-02 00:00:19,592 [DEBUG]: ----------------------------------------
2017-07-02 00:00:19,592 [DEBUG]: Error: Command 'git checkout --quiet c48bb8b02c98ae49438e43aa1143a958784822a5' returned non-zero exit status 128 in /home/clusterfuzz/chromium/src/third_party/swiftshader/third_party/pnacl-subzero
2017-07-02 00:00:19,592 [DEBUG]: fatal: reference is not a tree: c48bb8b02c98ae49438e43aa1143a958784822a5
2017-07-02 00:00:19,592 [DEBUG]: 
```

Since we don't fetch the history, the sha doesn't exist. I wonder if we should fetch the specific sha if it doesn't exist locally.

--no-history --shallow would help with speed (I think). It'd be great if this can be improved.