Merge branch 'jc/remove-treesame-parent-in-simplify-merges'

The --simplify-merges logic did not cull irrelevant parents from a
merge that is otherwise not interesting with respect to the paths
we are following.

This touches a fairly core part of the revision traversal
infrastructure; even though I think this change is correct, please
report immediately if you find any unintended side effect.

* jc/remove-treesame-parent-in-simplify-merges:
  simplify-merges: drop merge from irrelevant side branch

revision.c

@@ -1970,6 +1970,22 @@ static struct merge_simplify_state *locate_simplify_state(struct rev_info *revs,
 	return st;
 }
 
+static void remove_treesame_parents(struct commit *commit)
+{
+	struct commit_list **pp, *p;
+
+	pp = &commit->parents;
+	while ((p = *pp) != NULL) {
+		struct commit *parent = p->item;
+		if (parent->object.flags & TREESAME) {
+			*pp = p->next;
+			free(p);
+			continue;
+		}
+		pp = &p->next;
+	}
+}
+
 static struct commit_list **simplify_one(struct rev_info *revs, struct commit *commit, struct commit_list **tail)
 {
 	struct commit_list *p;
@@ -2022,10 +2038,18 @@ static struct commit_list **simplify_one(struct rev_info *revs, struct commit *c
 		if (revs->first_parent_only)
 			break;
 	}
-	if (!revs->first_parent_only)
-		cnt = remove_duplicate_parents(commit);
-	else
+
+	if (revs->first_parent_only) {
 		cnt = 1;
+	} else {
+		/*
+		 * A merge with a tree-same parent is useless
+		 */
+		if (commit->parents && commit->parents->next)
+			remove_treesame_parents(commit);
+
+		cnt = remove_duplicate_parents(commit);
+	}
 
 	/*
 	 * It is possible that we are a merge and one side branch

t/t6012-rev-list-simplify.sh

@@ -56,7 +56,23 @@ test_expect_success setup '
 
 	echo "Final change" >file &&
 	test_tick && git commit -a -m "Final change" &&
-	note I
+	note I &&
+
+	git symbolic-ref HEAD refs/heads/unrelated &&
+	git rm -f "*" &&
+	echo "Unrelated branch" >side &&
+	git add side &&
+	test_tick && git commit -m "Side root" &&
+	note J &&
+
+	git checkout master &&
+	test_tick && git merge -m "Coolest" unrelated &&
+	note K &&
+
+	echo "Immaterial" >elif &&
+	git add elif &&
+	test_tick && git commit -m "Last" &&
+	note L
 '
 
 FMT='tformat:%P 	%H | %s'
@@ -79,10 +95,10 @@ check_result () {
 	'
 }
 
-check_result 'I H G F E D C B A' --full-history
-check_result 'I H E C B A' --full-history -- file
-check_result 'I H E C B A' --full-history --topo-order -- file
-check_result 'I H E C B A' --full-history --date-order -- file
+check_result 'L K J I H G F E D C B A' --full-history
+check_result 'K I H E C B A' --full-history -- file
+check_result 'K I H E C B A' --full-history --topo-order -- file
+check_result 'K I H E C B A' --full-history --date-order -- file
 check_result 'I E C B A' --simplify-merges -- file
 check_result 'I B A' -- file
 check_result 'I B A' --topo-order -- file