Summary:
The `b2x:rebase` part is just like a changegroup, but it has a different name, and that results in `debugbundle` not telling us what the contents are.
This updates `debugbundle` to the contents of `b2x:rebase` parts.
Reviewed By: farnz
Differential Revision: D15822108
fbshipit-source-id: 60c28e4d965dabb7a889a14c179c1b69d0c5196e