Introduce git index mutations to randomized collaboration test

The test now fails at the following seed:

```bash
SEED=850 ITERATIONS=1 OPERATIONS=131 cargo test --package=collab random
```
529ccbda3aAntonio Scandurra committed on 1/8/2023, 3:52:16 PMparent3cffee4
1 file changedLine totals unavailable