histedit: add an experimental base action This is a first (very simple) version of the histedit base action. It works well in common usecases like rebasing the whole stack and spliting the stack. I don't see any obvious edge cases - but probably there is more than one. That's why I want to keep it behind experimental.histeditng config knob for now. I think on knob for all new histedit behaviors is better because we will test all of them together and testers will need to turn it on only once to get all new nice things.

histedit: add abortdirty function Small helper function for aborting histedit when left with dirty working directory.

histedit: add forceother constraint For the future 'base' action in histedit we need a verification constraint which will not allow using this action with changes that are currently edited.

histedit: make verification configurable Before we can add a 'base' action to histedit need to change verification so that action can specify which steps of verification should run for it. Also it's everything we need for the exec and stop actions implementation. I thought about baking verification into each histedit action (so each of them is responsible for verifying its constraints) but it felt wrong because: - every action would need to know its context (eg. the list of all other actions) - a lot of duplicated work will be added - each action will iterate through all others - the steps of the verification would need to be extracted and named anyway in order to be reused The verifyrules function grows too big now. I plan to refator it in one of the next series.

paper: show current revision on file log page Most of the pages in paper (and coal) style show the current revision and its branch, tags and bookmarks. Let's also show all this on file log page.

merge.applyupdates: extend action queues with ones returned from mergestate These queues will always be empty at the moment -- we're going to fill them up in upcoming patches.