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

Johannes Sixt [off-list ref] writes:

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?

I think --cc is designed to naturally fall back to -p when there is
only one parent.  Use of both -p and --cc has also long been an
acceptable combination, and even if we say the later --cc overrides
-p, there is no reason not to show single parent patch here with
--cc.