bug#43016: replace-region-contents takes a lot of time when called from json-pretty-print-buffer

[Eli Zaretskii]

> From: Tassilo Horn <tsdh@gnu.org>
> Cc: laszlomail@protonmail.com,  43016@debbugs.gnu.org,  eggert@cs.ucla.edu
> Date: Mon, 24 Aug 2020 21:15:42 +0200
> 
> ljell is right, it seems to have to do with the non-ASCII characters.
> In my sample.json.gz from above, I've just replaced every "e" with an
> "Ê" (except in true/false literals).  When I prettify that, it aborts
> early (fast) just after 449 early_abort_tests.
> 
> So just replacing "e" with an "Ê" changed "compares in time with 300.000
> early_abort_tests" to "doesn't compare in time and makes only 449
> early_abort_tests in that time".  The only difference to ljell's test
> file is that with mine, there doesn't seem to be a big gap between the
> last early_abort_test returning false and the one returning true.

I guess it's somehow related to the fact that buffer_chars_equal is
optimized for pure-ASCII buffers, see how we compute a_unibyte and
b_unibyte members of the context struct.