The try slaves run 'git revert' to clean up from the previous run. This only runs 'git reset --hard', which does not remove any untracked files, so if a CL adds a new file it will still be present after 'gclient revert'. Adding git clean -f -d removes untracked files (-f) and directories (-d).

Review URL: https://chromiumcodereview.appspot.com/10986032

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@159020 0039d316-1c4b-4281-b951-d872f2087c98