Changes in the validation of UTF-8

[Daurnimator <quae@...>]

I noticed the new commit that adds support for longer (deprecated in
2003) utf8 sequences:
https://github.com/lua/lua/commit/1e0c73d5b643707335b06abd2546a83d9439d14c

I'm curious why this changed? It seems like a backwards step to me.