When a git-cache operation is interrupted, it can leave behind large
temporary pack files.  Over time, these pack files will fill up
disks.

R=agable@chromium.org,hinoka@chromium.org,iannucci@chromium.org
BUG=352268

Review URL: https://codereview.chromium.org/326203003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@276208 0039d316-1c4b-4281-b951-d872f2087c98