hg: show the correct message when cloning an LFS repo with extension disabled

The `extensions._disabledpaths()` doesn't handle fetching help from `__index__`,
so it returns an empty dictionary of paths.  That means None is always returned
from `extensions.disabled_help()` when embedding resources inside the pyoxidizer
or py2exe binary, regardless of the arg or if is an external extension stored in
the filesystem.  And that means wrongly telling the user with an explicitly
disabled LFS extension that it will be enabled locally upon cloning from an LFS
remote.  That causes test-lfs-serve.t:295 to fail.

This effectively reverts most of the rest of 843418dc0b1b, while keeping the
help text change in place (which was specifically identified as a problem).

mercurial/hg.py

@@ -728,7 +728,7 b' def clone('
             # data.
             createopts[b'lfs'] = True
 
-            if extensions.disabled_help(b'lfs'):
+            if b'lfs' in extensions.disabled():
                 ui.status(
                     _(
                         b'(remote is using large file support (lfs), but it is '