clone: handle empty config values in -c

"git clone --config" uses the following incantation to add an item to
a config file, instead of replacing an existing value:

	git_config_set_multivar_gently(key, value, "^$", 0)

As long as no existing value matches the regex ^$, that works as
intended and adds to the config.  When a value is empty, though, it
replaces the existing value.

Noticed while trying to set credential.helper during a clone to use a
specific helper without inheriting from ~/.gitconfig and
/etc/gitconfig.  That is, I ran

	git clone -c credential.helper= \
		-c credential.helper=myhelper \
		https://example.com/repo

intending to produce the configuration

	[credential]
		helper =
		helper = myhelper

Without this patch, the 'helper =' line is not included and the
credential helper from /etc/gitconfig gets used.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -755,7 +755,9 @@ static int checkout(int submodule_progress)
 
 static int write_one_config(const char *key, const char *value, void *data)
 {
-	return git_config_set_multivar_gently(key, value ? value : "true", "^$", 0);
+	return git_config_set_multivar_gently(key,
+					      value ? value : "true",
+					      CONFIG_REGEX_NONE, 0);
 }
 
 static void write_config(struct string_list *config)

t/t5611-clone-config.sh

@@ -19,6 +19,14 @@ test_expect_success 'clone -c can set multi-keys' '
 	test_cmp expect actual
 '
 
+test_expect_success 'clone -c can set multi-keys, including some empty' '
+	rm -rf child &&
+	git clone -c credential.helper= -c credential.helper=hi . child &&
+	printf "%s\n" "" hi >expect &&
+	git --git-dir=child/.git config --get-all credential.helper >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'clone -c without a value is boolean true' '
 	rm -rf child &&
 	git clone -c core.foo . child &&