I came up with a workaround for my particular problem. The solution is to export the revisions before and after the split, skipping the split revision and filtering the pre-split revisions:
(bzr fast-export -r1..11 ~/src/repo | bzr fast-import-filter -i subdir/ -; bzr fast-export -r13..19 ~/src/repo) | git fast-import
The resulting repo contains only the history related to the split-off subdir, which is what I wanted.
I came up with a workaround for my particular problem. The solution is to export the revisions before and after the split, skipping the split revision and filtering the pre-split revisions:
(bzr fast-export -r1..11 ~/src/repo | bzr fast-import-filter -i subdir/ -; bzr fast-export -r13..19 ~/src/repo) | git fast-import
The resulting repo contains only the history related to the split-off subdir, which is what I wanted.