Estimated SVN revision number for release?

[MichaelChirico]

AIUI r-core does a lot of cherry-picking to include/exclude certain features, so we (probably) can't say "Version 3.6.3 was precisely r8xxxx", but it may still be worthwhile to associate each release with a ~rough revision number.

Maybe the SVN revision at the timestamp in r_versions() would be a good enough proxy (as long as it's well-documented that this is more heuristic than canon)?