Speed up 'replace-buffer-contents' some more

Speed up 'replace-buffer-contents' some more

* src/editfns.c (EXTRA_CONTEXT_FIELDS): New members beg_a and beg_b.
(Freplace_buffer_contents): Set up ctx.beg_a and ctx.beg_b.
(buffer_chars_equal): Use ctx->beg_a and ctx->beg_b instead of
calling BUF_BEGV, which is expensive.  This speeds up the recipe
in bug#31888 by 30%.