`sync-pr-commit-title` and `wait-for-ci` sometimes don't appear #3041

I don't know how this happens exactly, but sometimes the update note (highlighted in yellow below) doesn't appear (and therefore the PR’s title isn't updated)

<img width="487" alt="" src="https://user-images.githubusercontent.com/1402241/80547454-85343380-89b8-11ea-8768-33604804020e.png">

Same goes for wait-ci:

<img width="750" alt="Screen Shot 2020-04-29 at 19 28 25" src="https://user-images.githubusercontent.com/1402241/80627053-a2681100-8a4f-11ea-8a69-341726f4b0cb.png">

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This might happen when GitHub updates the content for the merge box but we don't detect it. Maybe on-replaced-element.ts can help us detect this change.