exthelper: reintroduce the ability to register revsets
I think this is what Yuya and Boris agreed on. This happens *after* the
extsetup phase now (and after the _aftercallback notifications). But this is
trivial, mergeable between exthelper instances, and doesn't need to have the
extension name supplied when registering.
The test needed updating so that extsetup() takes a ui argument, as exthelper
isn't trying to be backward compatible with 1.3.1.