Summary: As it was noted in D4851138, writing to stdout may break extensions.
Test Plan: arc unit
Reviewers: #mercurial, simpkins
Reviewed By: simpkins
Subscribers: net-systems-diffs@fb.com, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4948924
Tasks: 17453793
Signature: t1:4948924:1493151885:4bcb971e65ac623aa5f91cf80fee5af6902c9226