win32: enable legacy I/O mode to fix missing pager output on Windows with py3

The equivalent interpreter option is set by wrapper.exe, but this *.bat file is
what gets installed in a venv.  Without this mode, any command that spins up a
pager has no output, unless the pager is explicitly disabled.  The variable is
set inside the `setlocal` scope to keep it from leaking into the environment
after the bat file exits.

We should probably still figure out how to ship a compiled hg.exe when
installing with `pip`, because the binary does other things like enable long
filename support.  But this avoids the dangerous and confusing lack of output in
the meantime.

Differential Revision: https://phab.mercurial-scm.org/D10354

contrib/win32/hg.bat

@@ -4,6 +4,8 b' rem Windows Driver script for Mercurial'
 setlocal
 set HG=%~f0
 
+set PYTHONLEGACYWINDOWSSTDIO=1
+
 rem Use a full path to Python (relative to this script) if it exists,
 rem as the standard Python install does not put python.exe on the PATH...
 rem Otherwise, expect that python.exe can be found on the PATH.