pager: do not start pager if `ui` has been `pushbuffer`-ed

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

--- a/mercurial/ui.py	Wed Aug 30 16:05:12 2017 -0700
+++ b/mercurial/ui.py	Wed Aug 30 14:04:55 2017 -0700
@@ -962,6 +962,7 @@
             # formatted() will need some adjustment.
             or not self.formatted()
             or self.plain()
+            or self._buffers
             # TODO: expose debugger-enabled on the UI object
             or '--debugger' in pycompat.sysargv):
             # We only want to paginate if the ui appears to be

--- a/tests/test-pager.t	Wed Aug 30 16:05:12 2017 -0700
+++ b/tests/test-pager.t	Wed Aug 30 14:04:55 2017 -0700
@@ -340,6 +340,20 @@
    9: a 9
   10: a 10
 
+During pushbuffer, pager should not start:
+  $ cat > $TESTTMP/pushbufferpager.py <<EOF
+  > def uisetup(ui):
+  >     ui.pushbuffer()
+  >     ui.pager('mycmd')
+  >     ui.write('content\n')
+  >     ui.write(ui.popbuffer())
+  > EOF
+
+  $ echo append >> a
+  $ hg --config extensions.pushbuffer=$TESTTMP/pushbufferpager.py status --color=off
+  content
+  paged! 'M a\n'
+
 Environment variables like LESS and LV are set automatically:
   $ cat > $TESTTMP/printlesslv.py <<EOF
   > from __future__ import absolute_import