Decrease count when reverting reviews.

[zjosua]

Description

Fixes #18. Using the review_did_undo hook. Didn't test the legacy hook.

Checklist:

• [x] I've read and understood the contribution guidelines
• [x] I've tested my changes against at least one of the following Anki builds:
  • [ ] Latest standard Anki 2.1 binary build [required for Anki-compatible 2.1 add-ons]
  • [ ] Latest alternative Anki 2.1 binary build
  • [x] Anki 2.1.24 running from source (9dcbb895)
• [ ] I've tested my changes on at least one of the following platforms:
  • [x] Linux, version: Debian bullseye
  • [ ] Windows, version:
  • [ ] macOS, version:
• [ ] My changes potentially affect non-desktop platforms, of which I've tested:
  • [ ] AnkiMobile, version:
  • [ ] AnkiDroid, version:
  • [ ] AnkiWeb

[glutanimate]

Makes sense, thanks! Will take another closer look at this past the 23.10 compat release.