encoding: alias cp65001 to utf-8 on Windows

As far as I can tell, cp65001 is the Windows name for UTF-8. I don't know
how different it is from the UTF-8, but Python 3 appears to have introduced
new codec for cp65001, so the alias is enabled only for Python 2.

https://bugs.python.org/issue13216

This patch is untested, but hopefully fixes the following issue.

https://bitbucket.org/tortoisehg/thg/issues/5127/

mercurial/encoding.py

@@ -72,6 +72,11 b' else:'
     '646': lambda: 'ascii',
     'ANSI_X3.4-1968': lambda: 'ascii',
 }
+# cp65001 is a Windows variant of utf-8, which isn't supported on Python 2.
+# No idea if it should be rewritten to the canonical name 'utf-8' on Python 3.
+# https://bugs.python.org/issue13216
+if pycompat.iswindows and not pycompat.ispy3:
+    _encodingfixers['cp65001'] = lambda: 'utf-8'
 
 try:
     encoding = environ.get("HGENCODING")