This helps prevent timeout when checking out large repositories (like blink)
using the git cache_dir.

R=iannucci@chromium.org, maruel@chromium.org
BUG=295119

Review URL: https://codereview.chromium.org/43273005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@231111 0039d316-1c4b-4281-b951-d872f2087c98