magit-run-git-gui-blame: by default don't require confirmation

Instead require a prefix argument to allow selecting another revision
and/or file.  This was requested in #787.

@aspiers Could you please give this a try.

> https://github.com/magit/magit/pull/787#issuecomment-24940093  @aspiers If the user wants to launch it on a different file and/or revision, a prefix argument is the standard emacs way for the user to indicate that.

That's not universally true, see e.g. `kill-buffer`. Actually I think it is more often *not* true. I don't really care either way except that by now this once simple function is quite ugly. Any second opinions on this?