codegraph with readable path

Summary:
For diff analysis with codegraph to work, the path stored in must be relative to the project root,
         so that graph_code.marshall can be shared amoung different users.

Test Plan: make test

Reviewers: pad, pieter, mathieubaudet

Reviewed By: pad

CC: platform-diffs@lists

Differential Revision: https://phabricator.fb.com/D901334