bzr-svn: informing bzr about old svn merges.

[Óscar Fuentes]

Some long-lived branches started before svn had merge-tracking
capabilites and hence the merge information was manually tracked. After
converting the repositories to bzr with `bzr svn-import', the divergence
point of those branches wrt `trunk' is shown to be at the beginning of
the branch, as if no merges had ever happened.

Is there a recommended way of making bzr aware of the last merged
revision, so the next `bzr merge' considers only the subsequent
revisions?