Say why a merge's changed files read as none on an error

internal/builder/check_merge.go

@@ -85,7 +85,7 @@ func changedByMerge(ctx context.Context, run Runner, tree string, m TrunkMerge)
 	}
 	out, err := run(ctx, tree, "git", "diff", "--name-only", "--no-renames", m.Trunk, m.Merge)
 	if err != nil {
-		return nil
+		return nil // empty-on-error: the caller keeps the pull request's own list of changed files, the safe one
 	}
 	return strings.Fields(out)
 }