[BUG] gitk deals badly with --not

When gitk is run with "<some refs> --not <some other refs>" args, things work fine until one tries to modify the view. There it considers "--not" to be an extra git-log argument on its own, and groups the negative refs together with the positive refs, which naturally gives completely wrong results.

Not sure what the best course of action would be to fix that:
* add negative refs the extra args could be an option, since those are added last to the commandline, but then the
  "edit" dialog becomes a bit strange, with refs listed at both ends of it
* add a dedicated field for negative refs
* convert them to the ^ref syntax

Either of those last 2 solution raise the concern of convertion between ^ and --not: in the case where we add a dedicated field, would there be any reason not to move ^refs to this field instead ?