added possibility to ignore MagicMirror repo in updatenotification (#3002)

was [requested in the
forum](https://forum.magicmirror.builders/topic/17519/updatenotification).

- added possibility to exclude MagicMirror Repo and renamed it from
`default` to `MagicMirror`
- improved getting `behind` in case a hard `git fetch` was already done
- removed test "excludes repo if refs don't match regex" because of
above improvement this case is obsolete
- improved `git fetch --dry-run` with `-n` option to exclude tags (noise
reduction)

modules/default/updatenotification/updatenotification.js

@@ -77,7 +77,7 @@ Module.register("updatenotification", {
 
 	addFilters() {
 		this.nunjucksEnvironment().addFilter("diffLink", (text, status) => {
-			if (status.module !== "default") {
+			if (status.module !== "MagicMirror") {
 				return text;
 			}