Fix tox failures when run in Git Bash
Fix tox failures when run in Git Bash

When MSYSTEM is set to "MINGW64", git for Windows prints paths like

  c:/Users/...

when I do

  git submodule -q foreach --recursive 'printf "$toplevel/$path\n"'

When MSYSTEM is unset, git for Windows prints paths like

  /c/Users/...

which confuses check-manifest in an unfixable way (e.g. consider msys
"mounts" like /tmp).

See #64.

mgedmin/check-manifest,mauritsvanrees/check-manifest