cmdutil: allow native string as input to check_at_most_one_arg()
We seem to always convert **opts args to use bytes as keys early on in
Mercurial core, but I'm not sure we have good reason to do that, and
not all extensions do that. It's therefore helpful to be able to pass
in a native string to check_at_most_one_arg().
Differential Revision: https://phab.mercurial-scm.org/D7699
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