Extract finding the commits merged into a local main branch

The commits list will need this to decide which branches to mark. It
renders the list again whenever the selection changes, so stop the walk
at merged commits. Their parents are merged too, and a local main branch
is usually merged itself, so there is mostly nothing left to walk.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pkg/gui/branchstack/branchstack.go

@@ -27,16 +27,13 @@ func BranchesBelow(
 	mainBranches []string,
 ) []*models.Branch {
 	branchesByTip := map[string][]*models.Branch{}
-	mainBranchTips := []string{}
 	for _, other := range branches {
-		if lo.Contains(mainBranches, other.Name) {
-			mainBranchTips = append(mainBranchTips, other.CommitHash)
-		} else if other.Name != branch.Name {
+		if !lo.Contains(mainBranches, other.Name) && other.Name != branch.Name {
 			branchesByTip[other.CommitHash] = append(branchesByTip[other.CommitHash], other)
 		}
 	}
 
-	mergedLocally := reachableCommits(commits, mainBranchTips)
+	mergedLocally := CommitsMergedIntoLocalMainBranch(commits, branches, mainBranches)
 
 	result := []*models.Branch{}
 	for _, commit := range commits {
@@ -49,29 +46,47 @@ func BranchesBelow(
 	return result
 }
 
-// Returns the hashes of those of the given commits that can be reached from
-// any of the given tips by following their parents
-func reachableCommits(commits []*models.Commit, tips []string) *set.Set[string] {
-	commitsByHash := map[string]*models.Commit{}
+// Returns the hashes of those of the given commits that a local main branch
+// contains, but whose status doesn't say that they are merged. The status
+// looks at the remote branch of a main branch if it has one, so these are
+// commits that were merged into a local main branch but not pushed yet.
+func CommitsMergedIntoLocalMainBranch(
+	commits []*models.Commit,
+	branches []*models.Branch,
+	mainBranches []string,
+) *set.Set[string] {
+	result := set.New[string]()
+
+	mainBranchTips := lo.FilterMap(branches, func(branch *models.Branch, _ int) (string, bool) {
+		return branch.CommitHash, lo.Contains(mainBranches, branch.Name)
+	})
+	if len(mainBranchTips) == 0 {
+		return result
+	}
+
+	// The parents of a merged commit are merged too, so the walk never needs
+	// to go past a merged commit. Usually the tip of a local main branch is
+	// merged itself, so there is nothing to walk; this keeps it cheap enough to
+	// do on every render of the commits list.
+	unmergedCommitsByHash := map[string]*models.Commit{}
 	for _, commit := range commits {
-		if !commit.IsTODO() {
-			commitsByHash[commit.Hash()] = commit
+		if !commit.IsTODO() && commit.Status != models.StatusMerged {
+			unmergedCommitsByHash[commit.Hash()] = commit
 		}
 	}
 
-	reachable := set.New[string]()
-	pending := append([]string{}, tips...)
+	pending := mainBranchTips
 	for len(pending) > 0 {
 		hash := pending[len(pending)-1]
 		pending = pending[:len(pending)-1]
 
-		commit, ok := commitsByHash[hash]
-		if !ok || reachable.Includes(hash) {
+		commit, ok := unmergedCommitsByHash[hash]
+		if !ok || result.Includes(hash) {
 			continue
 		}
-		reachable.Add(hash)
+		result.Add(hash)
 		pending = append(pending, commit.Parents()...)
 	}
 
-	return reachable
+	return result
 }