[Fitness functions] Support git merge on pre commit hook

When using pre-commit hooks, we currently use `git diff --cached HEAD` to get the diff.

When merging develop in, fixing conflicts, and then committing, the diff command fails with a circular reference error.

We should change the command so it supports both rebases and merges.

## Screenshots



Locate the pre-commit hook or fitness-function code that runs `git diff --cached HEAD`, then inspect how it behaves during a merge and a rebase. Reproduce the circular-reference failure and verify that the updated diff command works for both workflows without breaking the existing hook behavior.

Written by the indexing model from the issue text.