Summary:
Moves ctreemanifest into hgext/extlib/. D6679698 was committed to scratch branch
by mistake.
Test Plan: make local && cd tests && ./run-tests.py
Reviewers: durham, #mercurial, #sourcecontrol
Reviewed By: durham
Differential Revision: https://phabricator.intern.facebook.com/D6684623
Signature: 6684623:1515522634:9bec363d00990d9ff7d5f655e30ab8cae636155c
