Bzr revision metadata that doesn't exist in git:

 - revision ids
  * one-on-one mapping to git-sha's and revision properties
 - file ids
  * git-sha+path -> fileid mapping

"bzr" git branch with bzr-specific metadata:

 * under refs/remotes/ to hide it from the user
 * Tree object with several children:
  * ":revisions": single object with mappings from git sha to (bzr revid, revprops, file id mapping object)
  * "bzr revision id": single object with mappings from git sha to bzr file id, revision id

.bzr needs extra git metadata:

(for round-tripped revisions only)
 * global map from git sha's to bzr revision ids
  + generated by bzr-git (slow) when not present
 * per bzr-revision map from git sha to bzr file id, revision id