parse-options.c: make OPTION_COUNTUP respect "unspecified" values

OPT_COUNTUP() merely increments the counter upon --option, and resets it
to 0 upon --no-option, which means that there is no "unspecified" value
with which a client can initialize the counter to determine whether or
not --[no]-option was seen at all.

Make OPT_COUNTUP() treat any negative number as an "unspecified" value
to address this shortcoming. In particular, if a client initializes the
counter to -1, then if it is still -1 after parse_options(), then
neither --option nor --no-option was seen; if it is 0, then --no-option
was seen last, and if it is 1 or greater, than --option was seen last.

This change does not affect the behavior of existing clients because
they all use the initial value of 0 (or more).

Note that builtin/clean.c initializes the variable used with
OPT__FORCE (which uses OPT_COUNTUP()) to a negative value, but it is set
to either 0 or 1 by reading the configuration before the code calls
parse_options(), i.e. as far as parse_options() is concerned, the
initial value of the variable is not negative.

To test this behavior, in test-parse-options.c, "verbose" is set to
"unspecified" while quiet is set to 0 which will test the new behavior
with all sets of values.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -110,6 +110,8 @@ static int get_value(struct parse_opt_ctx_t *p,
 		return 0;
 
 	case OPTION_COUNTUP:
+		if (*(int *)opt->value < 0)
+			*(int *)opt->value = 0;
 		*(int *)opt->value = unset ? 0 : *(int *)opt->value + 1;
 		return 0;