Re-extract the title/description at time of merge

[pnkfelix]

Spawned off of #51.

Homu is smart enough to update PR title/descriptions in response to new comments that are added.

However, if someone edits the title/description but does not add a new comment, homu won't see the edit.

This means that important edits like "[breaking-change]" or "this needs to go in beta channel too" are sometimes left out of the final commit record, and thus there is no record of the note except in the original Pull Request.

It seems like it would not be too much overhead for Homu to do one final extraction of the PR title/description at the time it constructs the merge commit to be tested.