Pull request edits should add further edits to the same PR.

[icculus]

The idea:

• When someone makes an untrusted edit, we make a topic branch (edit-$USERNAME-$TIMESTAMP) commit there and push it and generate a pull request for it.
• But if they make further edits (fix a typo, add other things), they have to edit the original text again and generate a second PR.
• We should instead make a single topic branch (edit-$USERNAME-$PAGENAME) if it doesn't already exist, and add more commits to it as they edit futher. As these commits are pushed, they just update the existing PR on GitHub, so you can see the revision history inline, and either rebase (several changes in a row that are unique improvements) or squash it (several typo fixes for the original changes) as appropriate.
• When merged, the topic branch is deleted, so future edits will make a new PR.
• When they edit, we start them from the latest in their topic branch instead of the latest in the wiki, so they can edit incrementally without having to redo all their changes to fix an additional typo.