diff-index: restore -c/--cc options handling
This fixes
19b2517f (diff-merges: move specific diff-index "-m"
handling to diff-index, 2021-05-21).
That commit disabled handling of all diff for merges options in
diff-index on an assumption that they are unused. However, it later
appeared that -c and --cc, even though undocumented and not being
covered by tests, happen to have had particular effect on diff-index
output.
Restore original -c/--cc options handling by diff-index.
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>