diff-index --cc no longer permitted, gitk is now broken (slightly)

Since 19b2517f95a0 (diff-merges: move specific diff-index "-m" handling
to diff-index, 2021-05-21) git diff-index no longer accepts --cc. This
breaks gitk: it invokes

   git diff-index --cached -p -C --cc --no-commit-id -U3 HEAD

to show the staged changes (when the line "Local changes checked in to
index but not committed" is selected).

The man page of git diff-index does not mention --cc as an option. I
haven't fully grokked the meaning of --cc, so I cannot tell whether this
absence has any significance (is deliberate or an omission).

Is gitk wrong to add --cc unconditionally? Should it do so only when
there are conflicts? Or not at all?

-- Hannes