[Follow-up] Handle description change and task note edge case

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Follow-up from "Prevent Description Change Notes When Toggling Tasks"

The following discussion from gitlab-ce!12057 should be addressed:

• @deckar01 started a discussion:

  It is possible for a user to edit the description text and modify a task item, which should theoretically generate both notes.

Proposal

1. Complete gitlab-ce#21590 so that the task summary is correctly parsed from the HTML (instead of markdown and regex).
2. Compare the number of tasks updated to the number of added/changed lines in the description diff.