## Summary

This PR improves the documentation for diff hunk keyboard shortcuts to
make existing functionality more discoverable to users.

### Problem

Users were unaware that pressing **ESC** already collapses all expanded
diff hunks through the existing `Cancel` action. The functionality
exists but lacks discoverability:

- The `Cancel` action description was too generic: "Cancels the current
operation"
- No documentation existed for diff hunk keyboard shortcuts in the Git
docs
- Users would naturally expect a dedicated keybinding for collapsing
diff hunks

Release Notes:

- N/A