bug#16546: C-s vs. whitespace highlighting

[Juri Linkov]

> IMO, Emacs should refrain from making permanent changes to the search
> string, as given by the user.  IOW, the search string should be kept
> untouched, and any transformations of it should be done "on the fly"
> to achieve the intended behavior, based on both the _current_ search
> string and the _current_ user options that govern the search behavior.

I agree that we have to keep the original string, and interpret it
differently according to the current search options.

And to distinguish the case when the search string was entered manually
and the case when the search string was yanked with `C-s C-y' I think
we should inspect the stack of `isearch-cmds'.  If it contains two or more
manually entered space characters then preserve them while matching whitespace.