Avoid unused line statistics when refreshing merge facts

internal/agents/mergefacts.go

@@ -123,11 +123,10 @@ func (m *Manager) gitFacts(ctx context.Context, t api.Task, p api.Project) (stor
 	if err != nil {
 		return g, err
 	}
-	all, err := git.Compare(t.WorktreePath, p.DefaultBranch, "HEAD", tree, "")
+	g.UnmergedFiles, err = git.ChangedFileCount(t.WorktreePath, p.DefaultBranch, tree)
 	if err != nil {
 		return g, err
 	}
-	g.UnmergedFiles = len(all.Files)
 	latest, err := m.store.LastReviewable(ctx, t.Id)
 	if errors.Is(err, store.ErrNotFound) {
 		return g, nil

internal/git/changes.go

@@ -43,6 +43,18 @@ func Compare(dir, defaultBranch, head, tree, since string) (api.TaskChanges, err
 	return api.TaskChanges{Base: base, Tree: tree, Files: files}, nil
 }
 
+func ChangedFileCount(dir, defaultBranch, tree string) (int, error) {
+	base, err := run(dir, "merge-base", "HEAD", defaultBranch)
+	if err != nil {
+		return 0, err
+	}
+	paths, err := run(dir, "diff", "--find-renames", "--name-only", "-z", base, tree)
+	if err != nil {
+		return 0, err
+	}
+	return strings.Count(paths, "\x00"), nil
+}
+
 func Snapshot(dir string) (string, error) {
 	index, err := run(dir, "rev-parse", "--path-format=absolute", "--git-path", "index")
 	if err != nil {