support rollback prior to 2019/10/01

[mrpiggi]

Following example fails with current release as it relies on the robust definition of \end (precisely \csname end \endcsname) which was introduced with 2019/10/01

\begin{filecontents*}{\jobname-sub.tex}
\documentclass[\jobname]{subfiles}
\begin{document}
\jobname-sub: sub text
\end{document}
\end{filecontents*}

\RequirePackage[2019/10/00]{latexrelease}
\documentclass{minimal}
\usepackage{subfiles}
\begin{document}
text

\subfile{\jobname-sub}

even more text
\end{document}

This PR resolves this issue.