GitHub PR Add Comment - initial value behavior

[WebsByTodd]

Steps to reproduce:

1. Navigate to a PR and type into the "Add a comment" textarea
2. Refresh the page without submitting the comment

Expected behavior:

• The comment is still visible

Actual behavior:

• The textarea value is set by GitHub but is not visible in the HTML markup
• Overtype preview is empty

Bandaid:

• Call updatePreview on every OverTypeInstance a few times during page-load