vim-patch:8.0.1763: :argedit does not reuse an empty unnamed buffer

Problem:    :argedit does not reuse an empty unnamed buffer.
Solution:   Add the BLN_CURBUF flag and fix all the side effects. (Christian Brabandt)

46a53dfc29

src/nvim/buffer.c

@@ -1717,11 +1717,7 @@ buf_T * buflist_new(char_u *ffname, char_u *sfname, linenr_T lnum, int flags)
    * buffer.)
    */
   buf = NULL;
-  if ((flags & BLN_CURBUF)
-      && curbuf != NULL
-      && curbuf->b_ffname == NULL
-      && curbuf->b_nwindows <= 1
-      && (curbuf->b_ml.ml_mfp == NULL || BUFEMPTY())) {
+  if ((flags & BLN_CURBUF) && curbuf_reusable()) {
     buf = curbuf;
     /* It's like this buffer is deleted.  Watch out for autocommands that
      * change curbuf!  If that happens, allocate a new buffer anyway. */
@@ -1864,6 +1860,17 @@ buf_T * buflist_new(char_u *ffname, char_u *sfname, linenr_T lnum, int flags)
   return buf;
 }
 
+/// Return true if the current buffer is empty, unnamed, unmodified and used in
+/// only one window. That means it can be reused.
+bool curbuf_reusable(void)
+{
+  return (curbuf != NULL
+          && curbuf->b_ffname == NULL
+          && curbuf->b_nwindows <= 1
+          && (curbuf->b_ml.ml_mfp == NULL || BUFEMPTY())
+          && !curbufIsChanged());
+}
+
 /*
  * Free the memory for the options of a buffer.
  * If "free_p_ff" is true also free 'fileformat', 'buftype' and