_find_diagonals which is only called if a file has a lot of changes would not work due to missing self, and cost parameters. self should be there since it is in a class, and is called via self._find... And cost is needed for some of the calculations and should have been passed as a parameter.

These changes are needed to allow the diff viewer to work with files with a large amount of change (200+ changes)

Tested a diff with 1280 changes, before the fix it would cause a crash/traceback to line 371, with _find_diagonal takes 11 param, found 12. After this fix, the diff viewer works as expect. Tested with 200+ reviews, and 800+ files, used in our testing system.