rebase: extract function for rebasing a single node We currently have _performrebase() and _performrebasesubset(), but we don't have a method for rebasing a single node (that's inside a loop in _performrebasesubset()). I think it makes sense to have such a method, so that's what this patch does. I think it may simplify future patches I'm working on that have to do with transactions, but I think this patch makes sense on its own whether or not that future work happens. Differential Revision: https://phab.mercurial-scm.org/D2809

rebase: use configoverride context manager for ui.forcemerge Differential Revision: https://phab.mercurial-scm.org/D2761

rebase: also restore "ui.allowemptycommit" value It looks like this was lost when the code was converted to the ui.configoverride() context manager in f255b1811f5e (rebase: get rid of ui.backupconfig, 2017-03-16). (And then the bad example was duplicated in 228916ca12b5 (rebase: add concludememorynode(), and call it when rebasing in-memory, 2017-12-07).) Differential Revision: https://phab.mercurial-scm.org/D2760

rebase: fix issue 5494 also with --collapse Differential Revision: https://phab.mercurial-scm.org/D2759