hash-object: replace stdin parsing OPT_BOOLEAN by OPT_COUNTUP

This task emerged from b04ba2bb (parse-options: deprecate OPT_BOOLEAN,
2011-09-27). hash-object is a plumbing layer command, so better
not change the input/output behavior for now.

Unfortunately we have these lines relying on the count up mechanism of
OPT_BOOLEAN:

	if (hashstdin > 1)
		errstr = "Multiple --stdin arguments are not supported";

Using OPT_BOOL will make "git hash-object --stdin --stdin" the same
as "git hash-object --stdin", resulting in just one object, which
will surprise users with an expectation to see two objects hashed.

Because it is not good to silently succeed and give an unexpected
result, even when the expectation is unrealistic, we use COUNTUP to
explicitly catch such an error.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>