String equals may be faster than Repository equals, id should never be null...

String equals may be faster than Repository equals, id should never be null and returning "null" doesn't seem useful...

src/de/uni_passau/fim/seibt/gitwrapper/repo/Reference.java

@@ -124,16 +124,16 @@ public abstract class Reference {
         }
 
         Commit commit = (Commit) o;
-        return Objects.equals(repo, commit.repo) && Objects.equals(id, commit.id);
+        return Objects.equals(id, commit.id) && Objects.equals(repo, commit.repo);
     }
 
     @Override
     public final int hashCode() {
-        return Objects.hash(repo, id);
+        return Objects.hash(id, repo);
     }
 
     @Override
     public String toString() {
-        return String.valueOf(id);
+        return id;
     }
 }