Apply phutil_utf8ize() to stderr output from VCS commands prior to logging

Summary: Ref T10228. Commands like git-http-backend can emit errors with raw bytes in the output. Sanitize these if present so we can log them in JSON format.

Test Plan: Edited this into production. >_> sneaky sneaky <_<

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10228

Differential Revision: https://secure.phabricator.com/D15144