We don't want to do a shallow clone into the cache because it is not supported
on versions of git older than 1.9.

BUG=261741

Review URL: https://codereview.chromium.org/198103012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@257578 0039d316-1c4b-4281-b951-d872f2087c98