PROCTools: More improvements to update references script
In !62281 (merged) I realised that the script didn't support:
- simultaneous digest and pool changes in one MR
- multi-line diff changes
This MR significantly simplifies diff handling (using the fact that event/run is a unique identifier and so we don't need more than this to identify which line to change) and now supports both of the above
cc @atlas-release-coordinators