rebase: use single transaction when running in memory rebase.singletransaction make rebase noticeably faster (~20% in a test I just ran). It is not enabled by default because it risks losing information if it aborts (see `hg help rebase`). When running rebase with the experimental in-memory option on, rebase is first attempted in memory, and if any conflicts occur, it restarts, this time writing to disk. Thus, it should be safe to turn on single-transaction mode for the in-memory phase. Differential Revision: https://phab.mercurial-scm.org/D3076