Get latest PR body before updating the PR body with links

[domenic]

Consider the following scenario, which occurs pretty often for HTML:

1. A PR is uploaded.
2. PR preview grabs the PR body text and commit contents.
3. Within the next 30 seconds, I edit the PR body text using GitHub's web UI
4. PR preview finishes building (~30 seconds later). It replaces the PR body text with the text from step (2) + its links to the preview. Thus, my edits are lost.

This problem is compounded if one is simultaneously pushing new commits and editing the PR body text in an intertwined fashion.

It would be ideal if once PR preview finished building in step 4, it grabbed a fresh copy of the PR body text to add its links to. This would greatly reduce the window in which edits could be lost: it would go down from "while the PR is being built" (which can be long) to "in between the fetch-new-body network request and the post-new-body network request".