Since some people have been asking about this... Here's how I'd see
upstream revision awareness being added to the -scm proposal.

* add src_fetch_extra or whatever to avoid doing the fetches in
src_unpack.

* add pkg_scm_info. It outputs a string containing no spaces.

* When installing an scm package with suitable EAPI to VDB / exndbam,
rewrite the scm version to -scm@THE_INFO_OUTPUT.

* When doing a pretend install, consider reinstalling any scm package
that was installed more than $user_option ago, and mark it as "might
reinstall".

* When updating an scm package, do src_fetch_extra then pkg_scm_info.
At user option, if the pkg_scm_info value hasn't changed and it's a
reinstall, skip reinstalling.

* At user option, and not by default, do the fetch / info stage
*before* showing the "this is what we'll install" list.

-- 
Ciaran McCreesh