Share commit preparation and checkout updates across merge paths

internal/agents/mergemain.go

@@ -116,12 +116,8 @@ func (m *Manager) pushToMain(ctx context.Context, l *live, task api.Task, projec
 		return "", err
 	}
 
-	coAuthors, err := m.CoAuthors(ctx, task.Id, user.Login)
-	if err != nil {
-		return "", err
-	}
-	env := git.IdentityEnv(user.GitName, user.GitEmail)
-	commit, err := git.SquashCommit(project.RepoPath, l.dir, project.DefaultBranch, squashMessage(title, coAuthors), env, m.signer(ctx, user))
+	task.MergeTitle = title
+	commit, notes, err := m.prepareMainCommit(ctx, task, project, user)
 	conflict := &git.ConflictError{}
 	if errors.As(err, &conflict) {
 		return "", fmt.Errorf("%w; run `git merge --no-ff --no-commit %s`, resolve them, and call merge_to_main again", err, project.DefaultBranch)
@@ -134,15 +130,6 @@ func (m *Manager) pushToMain(ctx context.Context, l *live, task api.Task, projec
 		return "", err
 	}
 
-	task.MergeTitle = title
-	note, err := m.mergeNote(ctx, task, user.Login)
-	if err != nil {
-		return "", err
-	}
-	notes, err := git.NoteCommit(project.RepoPath, commit, note, env)
-	if err != nil {
-		return "", err
-	}
 	err = git.Push(ctx, project.RepoPath, commit, project.DefaultBranch, notes)
 	if errors.Is(err, git.ErrRemoteMoved) {
 		return "", fmt.Errorf("%w; call merge_to_main again", err)
@@ -152,13 +139,9 @@ func (m *Manager) pushToMain(ctx context.Context, l *live, task api.Task, projec
 	}
 
 	pushed := git.Remote + "/" + project.DefaultBranch
-	err = git.AdvanceNotes(project.RepoPath, notes)
+	err = advanceMainCheckout(project, commit, notes)
 	if err != nil {
-		return "", fmt.Errorf("pushed %s to %s, but updating %s in the project checkout failed: %w", commit, pushed, git.NotesRef, err)
-	}
-	err = git.FastForward(project.RepoPath, commit)
-	if err != nil {
-		return "", fmt.Errorf("pushed %s to %s, but fast-forwarding the project checkout failed: %w", commit, pushed, err)
+		return "", err
 	}
 	err = git.Merge(l.dir, commit, fmt.Sprintf("Merge %s from %s", commit[:7], project.DefaultBranch), l.gitName, l.gitEmail)
 	if err != nil {