pager: do not start pager if `ui` has been `pushbuffer`-ed The `pushbuffer`, `popbuffer` APIs are intended to capture internal output. They will prevent `ui.write` from writing to the actual `ui.fout`. So a pager won't receive the output and do the right thing. In general, it does not make sense to start a pager if ui is in the "pushbuffer" mode. Differential Revision: https://phab.mercurial-scm.org/D574