commands: switch one call of check_at_most_one_arg to strings

This opts hasn't been through the byteskwargs mulcher, so we can just use
strings here instead of bytes. Fixes the test changes from D8204 on Python 3,
which was the only place this was a problem.

Differential Revision: https://phab.mercurial-scm.org/D8222

mercurial/commands.py

@@ -7665,7 +7665,7 b' def update(ui, repo, node=None, **opts):'
 
     Returns 0 on success, 1 if there are unresolved files.
     """
-    cmdutil.check_at_most_one_arg(opts, b'clean', b'check', b'merge')
+    cmdutil.check_at_most_one_arg(opts, 'clean', 'check', 'merge')
     rev = opts.get('rev')
     date = opts.get('date')
     clean = opts.get('clean')