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

extern crate cc;

fn main() {
    cc::Build::new()
        .warnings(true)
        .file("src/sendfds.c")
        .file("src/sighandlers.c")
        .compile("procutil");
}