exthelper: reintroduce the ability to register revsets
I think this is what Yuya and Boris agreed on.[1] 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.
[1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-December/125888.html
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