Create a wrapper for MSYS2 bash CI environment

This way there's no need to prepend every line in build job with a path
to MSYS2-installed bash, and deal with problems related to escaping
embedded shell invocations.

scripts/msys-bash.py

@@ -0,0 +1,42 @@
+#!/usr/bin/python3
+
+# Copyright (c) 2019 Patryk Obara <patryk.obara@gmail.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# pylint: disable=invalid-name
+# pylint: disable=line-too-long
+
+"""
+This is a wrapper for MSYS2 bash inside Windows GitHub CI environment.
+
+Usage:
+
+- name:  Step Name
+  shell: python scripts\msys-bash.py {0}
+  run:   echo "Hello"
+
+GitHub documentation for specifying shell is here: [1], although it
+leaves several VERY important details out:
+
+- The first parameter (program name) in template shell invocation needs to be
+  in PATH; absolute path will result in obscure C# exception.
+- Name of temporary script file ends with a hardcoded extension matching the
+  first parameter.
+- There is some additional undocumented trickery about expansion of {0} in
+  template (most probably it's something about formatting strings in C#).
+- For some shells, an additional first line is prepended to the temporary
+  script (but not for python!)
+
+[1] https://help.github.com/en/github/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#using-a-specific-shell
+"""
+
+import shutil
+import subprocess
+import sys
+
+if __name__ == "__main__":
+    script_py = sys.argv[1]
+    script_sh = script_py + '.sh'
+    shutil.copy(script_py, script_sh)
+    bash = [r'C:\tools\msys64\usr\bin\bash', '-eo', 'pipefail', '-l']
+    subprocess.call(bash + [script_sh])