Phillip Wood [off-list ref] writes:

"git pull" already runs "git merge-base --fork-point" before it runs 
"git fetch". The problematic reflog entry comes from a previous push 
which pushes to a different server due to remote.<remote>.pushurl. 

Ah, of course.  fork-point heuristics with a repository you yourself
push into would not make all that sense, since you are in control
when and what to push there in the first place :/.

Because we've just successfully pushed the local branch the fork point 
calculation thinks the remote tracking branch matches the local branch 
and so excludes all the local commits when we rebase but we didn't push 
it to the same server that we're fetching from. I wonder if we should 
disable the fork point calculation when there is a pushurl set.

Tempting thought.  Or educate users with diagnoses and advise()?