patch 8.2.5009: fold may not be closeable after appending

Problem:    Fold may not be closeable after appending.
Solution:   Set the fd_small flag to MAYBE. (Brandon Simmons, closes #10471)
diff --git a/src/fold.c b/src/fold.c
index 26a4a98..ee94a97 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -2574,6 +2574,7 @@
 					(long)(fp->fd_top - firstlnum));
 			    fp->fd_len += fp->fd_top - firstlnum;
 			    fp->fd_top = firstlnum;
+			    fp->fd_small = MAYBE;
 			    fold_changed = TRUE;
 			}
 			else if ((flp->start != 0 && lvl == level)