Removes DiffItem, which was unused, and moves _DiffItemInternal below GenerateDiff, now its lone caller.

No other code changes, so should be no behavioral change.

Review URL: https://chromiumcodereview.appspot.com/14241038

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@195262 0039d316-1c4b-4281-b951-d872f2087c98