RevisionMetadata._set_direct_lhs_parent_revmeta asserts while pulling ack's trunk

bzr.dev r3978, bzr-svn 0.5 r2514, subvertpy r2016.

I tried to pull a copy of http://ack.googlecode.com/svn/trunk that I used to pull with bzr-svn 0.4, but it bombed with an AssertionError. Deleting svn-cache didn't help. (Well, I didn't delete it. I just moved cache-v5 and parentmap-v1.* out of the way one at a time.) I didn't try a new bzr repo.

AssertionError: Tried registering <RevisionMetadata for revision 691, path trunk in repository '0c8536a8-3c1a-0410-a247-91dcf638db7e'> as parent while <RevisionMetadata for revision 692, path trunk in repository '0c8536a8-3c1a-0410-a247-91dcf638db7e'> already was parent for <RevisionMetadata for revision 693, path trunk in repository '0c8536a8-3c1a-0410-a247-91dcf638db7e'>