The options callback for --batch and --batch-check detects when the two
mutually incompatible options are used. But it simply returns an error
code to parse-options, meaning the program will quit without any kind of
message to the user.
Instead, let's use error() to print something and return -1. Note that
this flips the error return from 1 to -1, but negative values are more
idiomatic here (and parse-options treats them the same).
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>