If a user modifies a grep buffer, we can end up in weird situations
where we try match a filename over multiple lines.
Let's rule out newlines in filenames here. There is an argument
this is a case of GIGO but we already do this for the corresponding
highlighters.
We also do it in make.kak, see ca225ad4d (Cleanup make.kak and optimize
the make-next/make-prev regexes, 2016-12-09). There is one case left
where a filename would theoretically span multiple lines. Fix and
optimize this too.
40ffd5ff3b