Fix a couple bad fprintf() method calls in Phage

Summary: Ref T2794. These currently don't do the right thing if you run a command that prints "%" chacters.

Test Plan: Ran some command which outout "%" characters, no more warnings.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17385