vim-patch:9.1.0113: duplicate code when cleaning undo stack

Problem:  duplicate code when cleaning undo stack
Solution: refactor undo cleanup into a single public function

related: vim/vim#13928

9071ed8107

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/buffer.c

@@ -843,8 +843,9 @@ void buf_freeall(buf_T *buf, int flags)
   ml_close(buf, true);              // close and delete the memline/memfile
   buf->b_ml.ml_line_count = 0;      // no lines in buffer
   if ((flags & BFA_KEEP_UNDO) == 0) {
-    u_blockfree(buf);               // free the memory allocated for undo
-    u_clearall(buf);                // reset all undo information
+    // free the memory allocated for undo
+    // and reset all undo information
+    u_clearallandblockfree(buf);
   }
   syntax_clear(&buf->b_s);          // reset syntax info
   buf->b_flags &= ~BF_READERR;      // a read error is no longer relevant