Git Credential Manager for Windows (as of git 2.14.1) really wants to
pop up a modal dialog. Setting this environment variable avoids this
so that the check for src-internal access can quietly fail.

Ref: https://github.com/Microsoft/Git-Credential-Manager-for-Windows/issues/482

Bug: 755694
Change-Id: I38aec008662fa0a6bccb0a6220d376063ee790e7
Reviewed-on: https://chromium-review.googlesource.com/617502Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>