[cmf] handle xargs 123 return code for cmf --deadcode in hg repository

Summary: See the task

Test Plan: make

Reviewers: pieter, jcreed

Reviewed By: pieter

CC: bryano

Differential Revision: https://phabricator.fb.com/D1180771

Task ID: 3716802
++ -> @
* code_graph/model3.ml: misc