dirstate: generalize the dirstate's invalidation on transaction abort The previous code was too specific, we should add the invalidation at next to the `addfilegenerator` in the `write` call.

dirstate: simplify some methods' decorator Since `changing_parents` and `changing_files` are mutually exclusive, having both `@requires_changing_files` and `@requires_not_changing_parents` on a method is redundant.

dirstate: document the functions that need consolidation They are more functions that make the dirstate dirty but does not currently enforce a clean change scoping, we add comments in front of each of them so clarify this. There is no urgency to it, but the world will be a better place when all of them have proper scoping in place.

dirstate: make `restorebackup` more robust when it is a noop If there are no data_file, we might not be able to determine its filename. Which can be safely ignored. (all this code is on the way out anyway)