Add Support for MSYS2 in Windows

Thanks @asottile and all the other contributors for the great tool! ❤️ 👍

I am one of the maintainers for Gaphor, and we make heavy use of pre-commit as part of our work flow. It is also built in GTK, and we have to use MSYS2 to build the app for Windows. I know it is a pretty niche environment to support, but it would be really great if we could make it happen.

Current Behavior

version information

pre-commit version: 2.7.1
sys.version:
    3.8.5 (default, Sep  7 2020, 18:26:56)  [GCC 10.2.0 64 bit (AMD64)]
sys.executable: c:/tools/msys64/home/dyeaw/gaphor/.venv/bin/python.exe
os.name: nt
sys.platform: win32

error information

An unexpected error has occurred: FileNotFoundError: [WinError 3] The system cannot find the path specified: '/home/dyeaw/gaphor'

Traceback (most recent call last):
  File "C:/tools/msys64/home/dyeaw/gaphor/.venv/lib/python3.8/site-packages/pre_commit/error_handler.py", line 63, in error_handler
    yield
  File "C:/tools/msys64/home/dyeaw/gaphor/.venv/lib/python3.8/site-packages/pre_commit/main.py", line 343, in main
    _adjust_args_and_chdir(args)
  File "C:/tools/msys64/home/dyeaw/gaphor/.venv/lib/python3.8/site-packages/pre_commit/main.py", line 163, in _adjust_args_and_chdir
    os.chdir(toplevel)
FileNotFoundError: [WinError 3] The system cannot find the path specified: '/home/dyeaw/gaphor'

Updates Needed

• Update pre-commit to make use of the cygpath command to convert the path to the standard Windows path format after it gets the top-level path from git.
• Remove cygwin warnings about incompatible Python+git versions if in the MSYS2 environment
• Make upstream updates to setuptools_scm to patch additional path conversions for similar things as 1 above.

My plan, as long is everyone is good with adding support for this environment, is to submit a PR for the first two, and then I’ll start to work with the setuptools_scm team to see if I can fix the third one as well.

[asottile]

no problem! thanks for the helpful comment as well 😃

[danyeaw]

If anyone stumbles on this issue in the future and you want to run MINGW Python:

1. Remove msys git using pacman -R git
2. Install git-for-windows, if you use chocolately you can run choco install git
3. Add git to your path:

echo 'export PATH=$PATH:/c/ProgramData/chocolatey/bin' >> ~/.bashrc
source ~/.bashrc

Git-for-windows is compiled using MINGW64 as well, and returns the correct Windows paths. Thanks @asottile for the guidance, I don’t think this is well known for MSYS2 users, I’ll see if I can’t add it to their FAQ or other documentation. Sorry for being hard headed 😄.