[PATCH 0 of 2 RFC] Outdated Extension Detection

[Augie Fackler]

Right now this uses the part of the version number before the '+' to
check against declared-supported versions in an extension. I'm a
little nonplussed by how this breaks down for 'make local' copies of
Mercurial, but since that's an installation method that should
generally only be used by Users With Clue it seemed worth mailing this
out for comments.

I'm very open to comments on this, as well as better ways to detect
first vs third party extensions. It's worth noting that detecting
'hgext/' in the extension's __file__ won't work, since debian
maintainers of some of my third-party extensions have wedged them into
hgext (in at least one case after I complained to the maintainer about
this behavior).