Error out of the current tree is dirty (previously the dirty
content would be incorporated silently into the newly
squashed branch!).

Review URL: https://codereview.chromium.org/1064933004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294744 0039d316-1c4b-4281-b951-d872f2087c98