bug#40216: 28.0.50; Misinformation in isearch char-fold

[Juri Linkov]

> Out of curiosity, what were you searching for that resulted in such a
> large regexp?

Sometimes I pull a few of lines (usually 1-3 lines, not more)
from the buffer into the search string to confirm that the same lines
exist in more places in the same buffer ignoring the differences defined
by folding rules.  But after pulling 2 lines into the search string,
the generated regexp becomes so long that the regexp search fails
with the error "Regular expression too big".  Currently it silently
switches to literal search without notification that it doesn't follow
the folding rules anymore.  With the patch it informs about switching
to literal search.