After thinking more about it, the current change in output between the two
merges is not due to an ambiguity, it is introduce by a bug in merge/commit
that record a wrong file history.
Fixing the bug should fix the output. In the meantime, we have a wrong behavior.
In practice the bug is not in the merge code (who does the right things), but in
the commit code (who get confused when recording the new file revision).