Make -D -f combination more effective by deleting directories even if there is
modified files.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/4803001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@65845 0039d316-1c4b-4281-b951-d872f2087c98