hg-git part xi: merge day and non-fastforward commits

(you missed these, a tiny bit, didn't you.)

As I've mentioned here and here, non-fastforward commits are problematic for downstream partners, and are to be strictly avoided. And as I mentioned here, we had identified that our original merge day mechanics introduced non-fastforward commits to our Repositories of Record on a periodic basis.

I was under the impression that this was a Solved Problem, but I noticed a non-fastforward commit in mozilla-release after the mozilla-beta -> mozilla-release merge for Firefox 25.0 (thanks to the vcs-sync emails I'm getting now). I hadn't updated the instructions on the Merge Documentation wiki page, and the merge scripts followed that document.

The scripts are now available in this github repo. One pull request later, plus :lsblakk's fixes from the long Firefox 26 merge day, and we now use hg debugsetparents to "merge" in old heads in a fast-forwardable way. One more step towards our goal.