config: allow setting multivars when none exist yet

Adding a multivar when there are no variables with that name set
should set the variable instead of failing.

src/config_file.c

@@ -481,8 +481,10 @@ static int config_set_multivar(
 
 	pos = git_strmap_lookup_index(b->values, key);
 	if (!git_strmap_valid_index(b->values, pos)) {
+		/* If we don't have it, behave like a normal set */
+		result = config_set(cfg, name, value);
 		git__free(key);
-		return GIT_ENOTFOUND;
+		return result;
 	}
 
 	var = git_strmap_value_at(b->values, pos);

tests-clar/config/multivar.c

@@ -97,6 +97,22 @@ void test_config_multivar__add(void)
 	git_config_free(cfg);
 }
 
+void test_config_multivar__add_new(void)
+{
+	const char *var = "a.brand.new";
+	git_config *cfg;
+	int n;
+
+	cl_git_pass(git_config_open_ondisk(&cfg, "config/config11"));
+
+	cl_git_pass(git_config_set_multivar(cfg, var, "", "variable"));
+	n = 0;
+	cl_git_pass(git_config_get_multivar(cfg, var, NULL, cb, &n));
+	cl_assert(n == 1);
+
+	git_config_free(cfg);
+}
+
 void test_config_multivar__replace(void)
 {
 	git_config *cfg;