[regression] Rebase onto previous revision carries merge-metadata

When I rebase onto a previous revision (say to port a bugfix from trunk tip to something mergable into a previous release), bzr keeps the removed revisions as merged revisions in the base revision of the rebase.

Ok, that's almost unintelligible, here's a testcase:

bzr init-repo .
bzr init trunk
cd trunk
echo A > test
echo B > test2
bzr add test test2
bzr commit -m "Initial commit"
cd ..
bzr branch trunk feature
cd feature
echo B >> test
bzr commit -m "Test should have a B too"
cd ../trunk/
bzr merge ../feature/
bzr commit -m "Merged feature branch"
echo C >> test2
bzr commit -m "Test2 should have a C too"
cd ..
bzr branch trunk rebase-test
cd rebase-test
bzr rebase --onto 1 -r 3..
bzr log -r 2 -n0

The final bzr log will show merge history on revision 2. This means attempting to merge "rebase-test" into trunk will revert revision 2 in trunk.

bzr uncommit; bzr revert --forget-merges; bzr commit
achieves what I want, but one can't do this when there is a revision 4 too.