This will avoid errors where _Clone is called for a new DEP, but where the DEP
refers to a branch commit, thus causing the clone to fail because it can't
checkout that commit (such as happens currently with 'fetch v8').

Review URL: https://codereview.chromium.org/307223006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@274347 0039d316-1c4b-4281-b951-d872f2087c98