Add Co-authored-by trailers to Merge's squash commit

internal/agents/mergein.go

@@ -100,7 +100,12 @@ func (m *Manager) squashAndClose(ctx context.Context, taskID, login string) erro
 		return err
 	}
 
-	commit, err := git.SquashCommit(project.RepoPath, task.WorktreePath, project.DefaultBranch, task.MergeTitle, []string{
+	coAuthors, err := m.CoAuthors(ctx, taskID, login)
+	if err != nil {
+		return err
+	}
+
+	commit, err := git.SquashCommit(project.RepoPath, task.WorktreePath, project.DefaultBranch, squashMessage(task.MergeTitle, coAuthors), []string{
 		"GIT_AUTHOR_NAME=" + user.GitName,
 		"GIT_AUTHOR_EMAIL=" + user.GitEmail,
 		"GIT_COMMITTER_NAME=" + user.GitName,
@@ -132,6 +137,35 @@ func (m *Manager) squashAndClose(ctx context.Context, taskID, login string) erro
 	return err
 }
 
+func (m *Manager) CoAuthors(ctx context.Context, taskID, login string) ([]api.Participant, error) {
+	participants, err := m.store.Participants(ctx, taskID)
+	if err != nil {
+		return nil, err
+	}
+
+	coAuthors := []api.Participant{}
+	for _, p := range participants {
+		if p.Login != login {
+			coAuthors = append(coAuthors, p)
+		}
+	}
+	return coAuthors, nil
+}
+
+func squashMessage(title string, coAuthors []api.Participant) string {
+	trailers := []string{}
+	for _, p := range coAuthors {
+		if p.GitName == "" || p.GitEmail == "" {
+			continue
+		}
+		trailers = append(trailers, fmt.Sprintf("Co-authored-by: %s <%s>", p.GitName, p.GitEmail))
+	}
+	if len(trailers) == 0 {
+		return title
+	}
+	return title + "\n\n" + strings.Join(trailers, "\n")
+}
+
 func (m *Manager) RemoveTask(ctx context.Context, task api.Task, project api.Project, actor string) error {
 	err := m.StopTask(ctx, task.Id, actor)
 	if err != nil {