broken generation of short revision hashes in test_perf.py
Short git revision hashes are generated by reducing the length of the long hash to 7 characters which working most of the time, but not in all cases.

Examples:

```
0958d5f7fff105a593108aeab81413551e5e63c2 -> 0958d5f (7 characters)
8625aaeefdc89549b9a1017efcee4abb68840b06 -> 8625aaee (8 characters)
```
