extensions: avoid including `__index__` in the disabled extension list

extensions: avoid including `__index__` in the disabled extension list

This generated module contains a dictionary of all bundled extension names and
their help for builds that cannot enumerate extensions in the filesystem.

The disabled list gets displayed in `hg help extensions`, and is also used by
`setup.py` to populate `__index__.py` when building.  I haven't seen it sneak
into either py2exe or PyOxidizer builds, but it does show up when running tests
locally after having created an installer.

Differential Revision: https://phab.mercurial-scm.org/D9544

--- a/mercurial/extensions.py	Wed Dec 09 18:21:16 2020 -0500
+++ b/mercurial/extensions.py	Tue Dec 08 10:51:05 2020 -0500
@@ -810,7 +810,7 @@
     exts = {}
     for name, path in pycompat.iteritems(paths):
         doc = _disabledhelp(path)
-        if doc:
+        if doc and name != b'__index__':
             exts[name] = doc.splitlines()[0]
 
     return exts