Currently `subst-char-in-region' calls `modify_text' on the entire text,
starting from the first match to the end of the text, which fires off
`before-change-functions' for (almost) every character in the region
it's called in.

The specific place where this caused a problem for me is while filling
paragraphs with latex fragments in org-mode.  Latex fragments are
overlays with the `modification-hooks' property set to delete the
overlay, so when a paragraph is filled, that calls
  (subst-char-in-region ?\n ?\s ...)
on the entire paragraph which then calls `modify_text' on the entire
paragraph (sans the first line), which causes all latex fragments to
disappear.

I'm assuming that there isn't any particular reason why `modify_text'
should be called on the whole paragraph, but if there is, then that's ok
too, I suppose.  I don't see any reason why this should be breaking
stuff, but I haven't actually checked.