test-hardlink: stop explicitly listing `undo.dirstate` The files is on its way out.

localrepo: enforce a clean dirstate when the transaction open This is important to simplify the backup process. This also seems like a quite reasonable expectation. See inline documentation for details.

dirstate: explicitly backup the datafile Since the transaction does not know about this file, we need to explicitly mark it for backup before touching it.

mq: write the dirstate before stripping I am not certain why it is dirty (probably the `repo.setparent` call a right before. However I know we need it flushed before the repository starts to be stripped.

dirstate: simplify the shelve hack to not go through the disk We already have the data in memory, so why not simply keep the data in memory? This avoid abusing the `savebackup/restorebackup` logic and will make our life easier.

dirstate: detect potential fishy transaction patterns while changing If we rely on the transaction more, we should be more careful about the transaction. Adding extra security seems reasonable.

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)