windows: add windows behavior on broken pager
Apparently, Windows has "better" behavior than Unix in this case. This is an
edge case that led me down a rabbit hole, only to find a bug in the Python
documentation... I am not planning on trying to reproduce the same behavior
on Unix systems since it's not really useful, but other people are welcome to!
Differential Revision: https://phab.mercurial-scm.org/D11000