This allows people to configure their gitcookies file to be
whereever they want. As long as it actually exists and has
credentials in it, we'll accept it, just like git itself.

Change-Id: I4aa4806ddca0e61b28b003b0d3bc486407c13ab4
Reviewed-on: https://chromium-review.googlesource.com/951917Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>