Add a new command 'replace-buffer-contents' that uses the Myers diff
algorithm to non-destructively replace the accessible portion of the
current buffer. The Myers algorithm is implemented in Gnulib.
* src/editfns.c (Freplace_buffer_contents): New command.
(set_bit, bit_is_set, buffer_chars_equal): New helper functions.
(syms_of_editfns): Define new command.
* test/src/editfns-tests.el (replace-buffer-contents-1)
(replace-buffer-contents-2): New unit tests.
* src/buffer.h (BUF_FETCH_CHAR_AS_MULTIBYTE): New helper macro.
* admin/merge-gnulib (GNULIB_MODULES): Add diffseq.h and minmax.h.
