Matthieu Moy [off-list ref] writes:
Not HEAD, but .git/rebase-merge/onto, i.e. the target of the rebase.
Ideally, I would have loved to see "rebasing master on origin/master",
but I do not think the target ref name is stored during rebase.
Perhaps do it with --format=%s then. It often is useless to know
only the commit object name (or the branch name for that matter, as
you wouldn't know what exact commit the branch tip happens to have)
you are rebasing onto.