On Jun 28, 2018, at 0:52, Eric Abrahamsen <eric@ericabrahamsen.net> wrote:

$ git diff HEAD@{1}

shows the difference between HEAD and the previous state of HEAD.

That was new to me -- thanks! Just for completeness' sake, the explicit
(non-relative) way of doing this would be to look at the top of the
fast-forward message and use the updated-from commit as an anchor:

From git.sv.gnu.org:/srv/git/emacs
  61f73703c7..ce54573dac  master     -> origin/master
  12c77f6918..9134c841f1  emacs-26   -> origin/emacs-26
Updating 61f73703c7..ce54573dac

$ git log 61f73703c7..