merge-base: Add a case for Parent + child

Author: vHanda

lib/merge_base.dart

@@ -106,8 +106,7 @@ extension MergeBase on GitRepository {
         });
 
         if (commits.length == 1) {
-          // FIXME: Wtf? Where are we stopping?
-          throw Exception('Stop?');
+          break;
         }
 
         seen.add(fromAncestor.hash);

test/merge_base_test.dart

@@ -101,6 +101,7 @@ var independentData = [
   Test(['C', 'G', 'dev', 'M', 'N'], ['C', 'G', 'dev'], 'AcrossCrossMerges'),
   Test(['A', 'A^', 'A', 'M', 'N'], ['A', 'N'], 'ChangingOrderRepetition'),
   Test(['A^^^', 'A^', 'A^^', 'A', 'N'], ['A', 'N'], 'ChangingOrder'),
+  Test(['A', 'CD1'], ['CD1'], 'ParentChild'),
 ];
 
 class ATest {