Fix a flaky gsubtreed test caused by git2.repo logging
Fix a flaky gsubtreed test caused by git2.repo logging

Sample flake: http://build.chromium.org/p/chromium.infra/builders/infra-continuous-mac-10.9-64/builds/832/steps/cipd%20-%20test%20packages%20integrity/logs/stdio

BUG=

Review URL: https://codereview.chromium.org/1325753006

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra