diff: report bogus input to -C/-M/-B

diff: report bogus input to -C/-M/-B

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>