setup: exclude the __index__ module from itself when generating

This module is generated on Windows to hold all of the extension names and the
help summaries, so that they are discoverable inside the py2exe zipfile.  The
problem is this file is generated by dumping the disabled list, and that list
comes from walking the filesystem.  So once an install from source into a
virtualenv created this module, then next build from source from that virtualenv
would also see __index__.py in the filesystem, and include it.  Clearly that's
wrong because this isn't a real extension, so just filter it from the list when
generating it.

The Mercurial installer was unaffected by this, but the TortoiseHg package was.
In the final package, `hg help -v extensions` and the panel of extensions both
showed it.

Differential Revision: https://phab.mercurial-scm.org/D8142

setup.py

@@ -665,7 +665,9 b' class buildhgextindex(Command):'
         # here no extension enabled, disabled() lists up everything
         code = (
             'import pprint; from mercurial import extensions; '
-            'pprint.pprint(extensions.disabled())'
+            'ext = extensions.disabled();'
+            'ext.pop("__index__", None);'
+            'pprint.pprint(ext)'
         )
         returncode, out, err = runcmd(
             [sys.executable, '-c', code], localhgenv()