pager: add a config knob to just globally turn off the pager

mercurial/help/pager.txt

@@ -26,3 +26,10 b' you can use --pager=<value>::'
   - require the pager: `yes` or `on`.
   - suppress the pager: `no` or `off` (any unrecognized value
   will also work).
+
+To globally turn off all attempts to use a pager, set::
+
+  [pager]
+  enable = false
+
+which will prevent the pager from running.

mercurial/ui.py

@@ -857,6 +857,7 b' class ui(object):'
         if (self._disablepager
             or self.pageractive
             or command in self.configlist('pager', 'ignore')
+            or not self.configbool('pager', 'enable', True)
             or not self.configbool('pager', 'attend-' + command, True)
             # TODO: if we want to allow HGPLAINEXCEPT=pager,
             # formatted() will need some adjustment.