vim-patch:8.1.2094: the fileio.c file is too big

Problem:    The fileio.c file is too big.
Solution:   Move buf_write() to bufwrite.c. (Yegappan Lakshmanan,
            closes vim/vim#4990)

c079f0fed1

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/diff.c

@@ -22,6 +22,7 @@
 #include "nvim/ascii.h"
 #include "nvim/autocmd.h"
 #include "nvim/buffer.h"
+#include "nvim/bufwrite.h"
 #include "nvim/change.h"
 #include "nvim/charset.h"
 #include "nvim/cursor.h"