Bug 1100925 - Mach now detects msys2. r=gps

Before, mach would only detect MINGW32 (used with msys). Now it also detects MINGW64 (used with msys2).

MozReview-Commit-ID: 6IrNNmkLKgB

--HG--
extra : rebase_source : e84ee16b5bbc1b6b113163ba5b61759f763645c1

python/mach/mach/mixin/process.py

@@ -30,7 +30,7 @@ else:
 
 _in_msys = False
 
-if os.environ.get('MSYSTEM', None) == 'MINGW32':
+if os.environ.get('MSYSTEM', None) in ('MINGW32', 'MINGW64'):
     _in_msys = True
 
     if not _current_shell.lower().endswith('.exe'):