tests: remove unnecessary --traceback argument

tag: disallow tagging the working directory It's kinda silly, but a clear error message is better than a stacktrace about subscripting `None` when trying to generate the default commit message. I'm surprised that `.revsingle(..).node()` returns None instead of `nodemod.wdirid`, but now there's a test to catch if this changes.

dirstate: remove the dedicated backup logic When alone, the dirstate can now take care of its commit/rollback pattern itself. When in a transaction, the transaction deal with commit/rollback pattern quite fine. Why did you have a dedicated backup logic?

localrepo: stop doing special dirstate backup at transaction open Since the dirstate writes are already managed by the transaction, we already do a backup of the dirstate when necessary (and even trigger one to keep `hg rollback` happy). We needs some special code to deal with the initial empty checkout, but it is not too complicated. Managing variable filename (as dirstate-v2 uses) at the "journalfile" level, is complex and fragile (which is consistent with the fact these files are not journal…). If we no longer do it, our life is significantly simpler. In some sense, we apply the xkcd-1134¹ solution to our savebackup/restorebackup problem. [1] https://xkcd.com/1134/ (the change to test-hardlink are expect as decreasing the number of duplicated backup drive the hardlink count down)

localrepo: "blindly" do a dirstate backup at the end of the transaction Having the file backup mechanism dealing with file backup as benefit. So lets move closer to that. The fact `hg rollback` even needs this is sad. I hope to have the time to implement one of the alternative soon.

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.