grep: error out if --untracked is used with --cached

grep: error out if --untracked is used with --cached

The options --untracked and --cached are not compatible, but if they are
used together, grep just silently ignores --cached and searches the
working tree. Error out, instead, to avoid any potential confusion.

Signed-off-by: Matheus Tavares <redacted>
Reviewed-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>