py3: make sure opts are passed and used correctly in help command

opts are converted back to bytes in help.help_() where they are used.
Before that it's ensured that we have a bytes value for keep variable.

mercurial/commands.py

@@ -2759,7 +2759,7 b' def help_(ui, name=None, **opts):'
     Returns 0 if successful.
     """
 
-    keep = opts.get('system') or []
+    keep = opts.get(r'system') or []
     if len(keep) == 0:
         if pycompat.sysplatform.startswith('win'):
             keep.append('windows')

mercurial/help.py

@@ -23,6 +23,7 b' from . import ('
     filemerge,
     fileset,
     minirst,
+    pycompat,
     revset,
     templatefilters,
     templatekw,
@@ -304,6 +305,7 b' def help_(ui, name, unknowncmd=False, fu'
     '''
 
     from . import commands # avoid cycle
+    opts = pycompat.byteskwargs(opts)
 
     def helpcmd(name, subtopic=None):
         try: