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.