credential-store.c: replace home_config_paths() with xdg_config_home()

Since only the xdg credentials file path is required, and
home_config_paths() is unable to construct the path ~/.git-credentials,
simplify the code by replacing home_config_paths() with
xdg_config_home().

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

credential-store.c

@@ -170,7 +170,7 @@ int main(int argc, char **argv)
 	} else {
 		if ((file = expand_user_path("~/.git-credentials")))
 			string_list_append_nodup(&fns, file);
-		home_config_paths(NULL, &file, "credentials");
+		file = xdg_config_home("credentials");
 		if (file)
 			string_list_append_nodup(&fns, file);
 	}