bug#31837: 26.1; replace-buffer-contents doesn't work if buffer has multibyte characters

[Eli Zaretskii <eliz@gnu.org>]

> From: Milan Stanojević <mstanojevic@janestreet.com>
> Date: Mon, 18 Jun 2018 16:29:41 -0400
> Cc: 31837@debbugs.gnu.org
> 
> If we do care about optimizing it, then it is better to check whether
> the buffer actually contains multibyte characters (I guess with
> BUF_Z(buf) == BUF_Z_BYTE(buf)) than using enable_multibyte_characters
> since the the former would catch strictly more cases when we don't
> need charpos to bytepos conversion.

No, testing BUF_Z vs BUF_Z_BYTE cannot possibly catch more cases,
because multibyte characters cannot happen in a buffer whose
enable_multibyte_characters is reset, and any unibyte buffer's Z value
is trivially equal to its Z_BYTE value.

> Also, it occurs to me, the check shouldn't be done in
> buffer_chars_equal, since the same check would be done multiple times
> for a single character (for every comparison of that character), it
> should be done once and the result stored in ctx before calling
> compareseq.

Something to consider if further optimization is needed.