Re: [PATCH] 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.