rebase: use one dirstateguard for when using rebase.singletransaction This was previously landed as 2519994d25ca but backed out in b63351f6a2 because it broke hooks mid-rebase and caused conflict resolution data loss in the event of unexpected exceptions. This new version adds the behavior back but behind a config flag, since the performance improvement is notable in large repositories. The old commit message was: Recently we switched rebases to run the entire rebase inside a single transaction, which dramatically improved the speed of rebases in repos with large working copies. Let's also move the dirstate into a single dirstateguard to get the same benefits. This let's us avoid serializing the dirstate after each commit. In a large repo, rebasing 27 commits is sped up by about 20%. I believe the test changes are because us touching the dirstate gave the transaction something to actually rollback. (grafted from 9e3dc3a1638b9754b58a0cb26aaa75d868058109) (grafted from 7d38b41d2266d9a02a15c64229fae0da5738dcec) Differential Revision: https://phab.mercurial-scm.org/D135