extensions: load help from hgext.__index__ as a fallback this time

Prior to 843418dc0b1b, `hgext.__index__` was consulted first if present, which
caused the longer help from the extension modules to be ignored, even when
available.  But that change causes a bunch of test failures when the pyoxidized
binary bundles *.pyc in the binary, saying the there's no help topic for
`hg help $disabled_extension` and suggesting the use of `--keyword`, rather than
showing a summary and indicating that it is disabled.  Current failures were in
test-check-help.t, test-extension.t, test-help.t, and test-qrecord.t.

Ideally, we would read the various *.pyc files from memory and slurp in the
docstring, but I know that they used to not be readable as resources, and I
can't figure out how to make it work now.  So maybe 3.9 and/or the current
PyOxidizer doesn't support it yet.  I got closer in py2exe with
`importlib.resources.open_binary("hgext", "rebase.pyc")`, but `open_binary()` on
*.pyc fails in pyoxidizer.[1]  Either way, the *.pyc can't be passed to
`ast.parse()` as `extensions._disabledcmdtable()` is doing, so I'm setting that
aside for now.

[1] https://github.com/indygreg/PyOxidizer/issues/649

mercurial/extensions.py

@@ -841,6 +841,22 b' def disabled_help(name):'
     paths = _disabledpaths()
     if name in paths:
         return _disabledhelp(paths[name])
+    else:
+        try:
+            import hgext
+            from hgext import __index__  # pytype: disable=import-error
+
+            # The extensions are filesystem based, so either an error occurred
+            # or all are enabled.
+            if util.safehasattr(hgext, '__file__'):
+                return
+
+            if name in _order:  # enabled
+                return
+            else:
+                return gettext(__index__.docs.get(name))
+        except (ImportError, AttributeError):
+            pass
 
 
 def _walkcommand(node):