merge.recordupdates: don't require action keys to be present in dict We're going to use this function for a much smaller set of actions in the next patch. It's easier to do this than to backfill the dict we pass in.

histedit: constant-ify the constraints list Used a class as a namespace, and then wired up a classmethod to return all known constraints. I'm mostly happy with this, even though it's kind of weird for hg.