git rebase -p does not find mainline

When doing git rebase -p in order to do a non-trivial rebase tracking merges, I get a message about a missing -m option for specifying the mainline.

git rebase does not have the corresponding option.  The fix is to do

git cherry-pick -m 1 offending-merge-commit git rebase --continue

but that is quite unobvious from the resulting error message that does not even mention "cherry-pick". Since git rebase -p is supposed to "preserve merges", it should just cherry-pick with the same mainline that the original merge commit had, without asking the user questions or even failing.

This is the version of git delivered with Ubuntu 11.10, namely 1.7.5.4.