Parse the whole buffer at once in compile.el

Parse the whole buffer at once in compile.el

* lisp/progmodes/compile.el (compilation-next-single-property-change):
Parse whole buffer at once (bug#42806).

Also remove the comment that mentioned that it is an option to do it
in one go as we now actually start doing.  As the existence of that
comment suggested, there is not really a reason to process the buffer
in small chunks.  On the contrary, processing the output in arbitrary
units can result in certain constructs not being recognized because
they begin in one arbitrary chunk, while ending in another.