Seems like some combination of things (bad cache dir, shallow checkout, etc) causes
git cache to not clean up properly before trying to move new directory A into
already existing directory B.

So the logic is all there to detect that the cache is invalid, its just not
being cleaned up properly during shallow checkouts.

BUG=406864
TBR=iannucci

Review URL: https://codereview.chromium.org/499123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291587 0039d316-1c4b-4281-b951-d872f2087c98