backout: migrate `opts` to native kwargs
It will take a bit to unwind `cmdutil.commit`, so there's a conversion to
byteskwargs there, without changing the type of `opts` in this function. That's
also useful to flag it as needing to be changed.
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif