We already detect invalid input to these functions, but we
simply exit with an error code, never saying anything as
simple as "your input was wrong". Let's fix that.
Before:
$ git diff -CM
$ echo $?
128
After:
$ git diff -CM
error: invalid argument to -C: M
$ echo $?
128
There should be no problems with having diff_opt_parse print
to stderr, as there is already precedent in complaining
about bogus --color and --output arguments.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>