magit-diff-with-mark: only one argument RANGE

Replace `magit-diff-with-mark's MARKED and COMMIT arguments with just
RANGE, making it consistent with other diff commands.  Keeping these
arguments for non-interactive use would not make sense since the marked
commit is determined inside `interactive'.  Except for that part it
behaves the same ad `magit-diff', the latter should therefor be used
non-interactively.