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

# See http://EditorConfig.org for the specification

root = true

[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true

[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true

[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false