show log of recent upgrade events

@jennybc says:

> I do a casual `install_github("this/that")` and find that the bleeding edge version of something I inadvertently updated is a bit broken and I want to roll it back. I wish it were easy to see a log of recent upgrade events with “X.Y.Z -> “X*.Y*.Z*” or “SHA_before --> SHA_after” type of info.

See also #3 