revert after merge doesn't delete directories added by merge itself

Reproduction of problem (in fresh bzr.dev):

$ bzr init a
$ cd a
$ bzr mkdir dir
added dir
$ cd dir
$ echo spam > spam
$ bzr add spam
added dir/spam
$ cd ..
$ bzr ci -m initial
added dir
added dir/spam
Committed revision 1.
$ cd ..
$ bzr branch a b
Branched 1 revision(s).
$ cd b
$ cd dir
$ bzr mkdir subdir
added subdir
$ cd subdir
$ echo foo > bar
$ bzr add bar
added dir/subdir/bar
$ cd ..
$ cd ..
$ bzr ci -m subdir
added dir/subdir
added dir/subdir/bar
Committed revision 2.
$ bzr ls
dir
dir/spam
dir/subdir
dir/subdir/bar
$ cd ..
$ cd a
$ bzr merge ../b
All changes applied successfully.
$ bzr st
added:
  dir/subdir/
  dir/subdir/bar
pending merges:
  Alexander Belchenko 2006-07-26 subdir
$ bzr revert
$ bzr st
unknown:
  dir/subdir
$ bzr ls
dir
dir/spam
dir/subdir

File 'dir/subdir/bar' is actually deleted but 'subdir' is not.