MSYSTEM is not passed by default on Windows

[mgedmin]

Summary

I discovered this when people started reporting breakage when they run check-manifest from tox, from a Git for Windows bash.exe shell: mgedmin/check-manifest#64

When you run git.exe on Windows, the way it prints filenames depends on the environment. If MSYSTEM is set to MINGW64, the filenames are printed in the standard Windows fashion (e.g. C:\Users\...). When MSYSTEM is not set, filenames are printed in the Unix fashion (/c/Users/...).

check-manifest runs git in a subprocess to enumerate submodules and versioned files. This works fine, except when tox clears the environment, things break.

I think MSYSTEM should be included in tox's default passenv list, since it describes system configuration (sort of like TERM, or LC_CTYPE) and a wrong value breaks things.

Steps to reproduce

• get a Windows VM
• install Git for Windows
• install Python
• install tox
• launch Git Bash
• git clone https://github.com/mgedmin/check-manifest
• cd check-manifest
• git checkout 0.34 # master has a workaround for this
• tox --develop -e py27 -- tests:TestGit.test_get_versioned_files_with_git_submodules

Expected output

• tox indicates it ran 1 test successfully

Actual output

• tox shows a test failure where subprocess.Popen(..., cwd=...) says the directory name is invalid.