(Freplace_buffer_contents): Preserve markers more carefully

(Freplace_buffer_contents): Preserve markers more carefully

Use `replace_range` rather than `delete+insert`.

* src/insdel.c (replace_range): Allow NEW to specify a chunk
of buffer text.
* src/editfns.c (Freplace_buffer_contents): Use it.
* test/src/editfns-tests.el (replace-buffer-contents-1): Remove
incorrect check which happened to succeed because point was not
preserved carefully enough.  Make the replacement text share a bit
more content to make the test a bit more strict.
(editfns-tests--replace-region): Doesn't fail any more.

(cherry picked from commit 40d8650d5177bd291747d225c7d2cf2b4ba55856)