help: fix a py3 error interpolating Set into b'%s'

I can't reproduce it, but a coworker hit this with `hg help -v` with 6.2.3:

    ...
      File "mercurial\help.pyc", line 865, in helplist
    TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'set'

I can confirm that the original expression fails in `hg debugshell`, and the new
one works.  The second instance was found by searching for "%s", but PyCharm
detects a lot of variables as Any type, so I have no idea if there are other
lurking problems.

mercurial/help.py

@@ -864,7 +864,7 b' def help_('
             if missing_order:
                 ui.develwarn(
                     b'help categories missing from CATEGORY_ORDER: %s'
-                    % missing_order
+                    % stringutil.forcebytestr(missing_order)
                 )
 
             # List per category.
@@ -897,7 +897,7 b' def help_('
             if missing_order:
                 ui.develwarn(
                     b'help categories missing from TOPIC_CATEGORY_ORDER: %s'
-                    % missing_order
+                    % stringutil.forcebytestr(missing_order)
                 )
 
             # Output topics per category.