changegroup: simplify logic around enabling changegroup 03
There was multiple spot that took care of adding '03' as supported changegroup
version for different condition. We gather them all in one location for
simplicity.
The 'supportedincomingversions' function is now doing nothing, but I kept it
around because it looks like a great hooking point for extension.
(Note that we should probably just get changegroup3 out of experimental now, But
that would be a patch with a much wider scope).
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors