cmdutil: add a kludge to make bytes repr() the same on 2 and 3
This fixes the output formatting problems I see in debugobsolete. I
still am seeing some effectflag differences, which we'll need to
tackle separately.
I'm not in love with this approach. There might be something better we
could do, and I'd love it if someone else wanted to take a run at
this.
Differential Revision: https://phab.mercurial-scm.org/D1909