Add command to replace buffer contents

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.

1 files changed, 5 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 2fb8daab101..ab600eb2786 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -462,6 +462,11 @@ Negative prefix arg flips the direction of selection.  Also,
 defun are selected unless they are separated from the defun by a blank
 line.
 
+** New command 'replace-buffer-contents'.  This command replaces the
+contents of theaccessible portion of the current buffer with the
+contents of the accessible portion of a different buffer while keeping
+point, mark, markers, and text properties as intact as possible.
+
 
 * Changes in Specialized Modes and Packages in Emacs 26.1