Test Plan: run tree_dump on a fastmanifest file. did some spot checks against the text manifest with the same sha (`hg debugdata -m`)
Reviewers: lcharignon, durham
Reviewed By: durham
Subscribers: mitrandir, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D3585843
Tasks: 12119301
Signature: t1:3585843:1469050470:70729d5601f935f2de80f89a7ad83590bcb8dd00