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