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