When the argument -- is passed to clang-tidy-diff.py it should pass the following arguments to clang-tidy.
It does that but also includes -- as an argument,
there should be a +1 in the '--' index.
So only the following arguments as passed to clang-tidy.
If we make the script leave out the -- flag, we should stop forwarding these flags and the extra_arg(_before)? below. Otherwise it's too confusing (should one place -fix before -- or after? what about -warnings-as-errors?).
Please also update the usage example at the top.