vim-patch:8.2.5009: fold may not be closeable after appending (#18722)

Problem:    Fold may not be closeable after appending.
Solution:   Set the fd_small flag to MAYBE. (Brandon Simmons, closes vim/vim#10471)
e8c4a64bff

src/nvim/fold.c

@@ -2323,6 +2323,7 @@ static linenr_T foldUpdateIEMSRecurse(garray_T *const gap, const int level,
               }
               fp->fd_len += fp->fd_top - firstlnum;
               fp->fd_top = firstlnum;
+              fp->fd_small = kNone;
               fold_changed = true;
             } else if ((flp->start != 0 && lvl == level)
                        || (firstlnum != startlnum)) {