If the Infra Git wrapper is managing a Git execution, "git_retry" will
impose a second set of retries. Recognize this situation and disable
"git_retry" so that the Git wrapper is the sole managing process.

BUG=chromium:703868
TEST=local
  - Ran with and without environment variable, seems to work.

Change-Id: I4f749c9693a761abe2a062ed5f1dc09f51c69e30
Reviewed-on: https://chromium-review.googlesource.com/469229
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>