Missing description. Stealing from the link you sent:
The typical use-case is starting a rebase, do something else, come back
the day after, run "git status" or make a new commit and wonder what
in the world's going on. Which branch is being rebased is probably the
most useful tidbit to help, but the target may help too.
Ideally, I would have loved to see "rebasing master on origin/master",
but the target ref name is not stored during rebase, so this patch
writes "rebasing master on a78c8c98b" as a half-measure to remind
future users of that potential improvement.
Signed-off-by: <...>
Jonathan Nieder <jrnieder@gmail.com>