histedit: add histedit.singletransaction config option This adds an option (which defaults to False) to run entire histedits in a single transaction. This results in 20-25% faster histedits in large repos where transaction startup cost is expensive. I didn't want to enable this by default because it has some unfortunate side effects. For instance, if a pretxncommit hook throws midway through the histedit, it will rollback the entire histedit and lose any progress the user had made. Same if the user aborts editting a commit message. It's still worth turning this on for large repos, but probably not for normal sized repos. Long term, once we have inmemory merging, we could do the entire histedit in memory, without a transaction, then we could selectively rollback just parts of it in the event of an exception. Tested it by running the tests with `--extra-config-opt=histedit.singletransaction=True`. The only failure was related to the hook rollback issue I mention above.

histedit: pop action after the action is completed We only want to pop the action after the action is completed, since if the action aborts part way through we want it to remain at the front of the list so continue/abort will start with it. Previously we relied on the fact that we only serialized the state file at the beginning of the action, so the pop wasn't serialized until the next iteration of the loop. In a future patch we will be adding a large transaction around this area, which means if we pop the list early it might get serialized if the action throws a user InterventionRequired error, at which point the action is not in the list anymore. So let's only pop it once the action is really truly done.

histedit: add transaction support to writing the state file This will be used in a future diff to enable a single transaction around an entire histedit.