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
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