blackbox: set lastui even if ui.log is not called (issue5518) `lastui` decides where (where is the `.hg`) to use if the current `ui` object does not have a `_bbrepo` associated. Previously it only gets set in `ui.log`, which means unless a `ui` with repo associated calls `log` with tracked event, blackbox does not know where to write its log. This patch makes `reposetup` set `lastui` so it so we could log some more events (see test changes). Differential Revision: https://phab.mercurial-scm.org/D655