config tests: add "NULL" tests for *_get_value_multi()

A less well known edge case in the config format is that keys can be
value-less, a shorthand syntax for "true" boolean keys. I.e. these two
are equivalent as far as "--type=bool" is concerned:

	[a]key
	[a]key = true

But as far as our parser is concerned the values for these two are
NULL, and "true". I.e. for a sequence like:

	[a]key=x
	[a]key
	[a]key=y

We get a "struct string_list" with "string" members with ".string"
values of:

	{ "x", NULL, "y" }

This behavior goes back to the initial implementation of
git_config_bool() in 17712991a5 (Add ".git/config" file parser,
2005-10-10).

When parts of the config_set API were tested for in [1] they didn't
add coverage for 3/4 of the "(NULL)" cases handled in
"t/helper/test-config.c". We'd test that case for "get_value", but not
"get_value_multi", "configset_get_value" and
"configset_get_value_multi".

We now cover all of those cases, which in turn expose the details of
how this part of the config API works.

1. 4c715ebb96 (test-config: add tests for the config_set API,
   2014-07-28)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1308-config-set.sh

@@ -146,6 +146,71 @@ test_expect_success 'find multiple values' '
 	check_config get_value_multi case.baz sam bat hask
 '
 
+test_NULL_in_multi () {
+	local op="$1" &&
+	local file="$2" &&
+
+	test_expect_success "$op: NULL value in config${file:+ in $file}" '
+		config="$file" &&
+		if test -z "$config"
+		then
+			config=.git/config &&
+			test_when_finished "mv $config.old $config" &&
+			mv "$config" "$config".old
+		fi &&
+
+		# Value-less in the middle of a list
+		cat >"$config" <<-\EOF &&
+		[a]key=x
+		[a]key
+		[a]key=y
+		EOF
+		case "$op" in
+		*_multi)
+			cat >expect <<-\EOF
+			x
+			(NULL)
+			y
+			EOF
+			;;
+		*)
+			cat >expect <<-\EOF
+			y
+			EOF
+			;;
+		esac &&
+		test-tool config "$op" a.key $file >actual &&
+		test_cmp expect actual &&
+
+		# Value-less at the end of a least
+		cat >"$config" <<-\EOF &&
+		[a]key=x
+		[a]key=y
+		[a]key
+		EOF
+		case "$op" in
+		*_multi)
+			cat >expect <<-\EOF
+			x
+			y
+			(NULL)
+			EOF
+			;;
+		*)
+			cat >expect <<-\EOF
+			(NULL)
+			EOF
+			;;
+		esac &&
+		test-tool config "$op" a.key $file >actual &&
+		test_cmp expect actual
+	'
+}
+
+test_NULL_in_multi "get_value_multi"
+test_NULL_in_multi "configset_get_value" "my.config"
+test_NULL_in_multi "configset_get_value_multi" "my.config"
+
 test_expect_success 'find value from a configset' '
 	cat >config2 <<-\EOF &&
 	[case]