patch 8.1.0515: reloading a script gives errors for existing functions

Problem:    Reloading a script gives errors for existing functions.
Solution:   Allow redefining a function once when reloading a script.
diff --git a/src/buffer.c b/src/buffer.c
index cbccc70..8e892da 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5519,6 +5519,7 @@
 #ifdef FEAT_EVAL
 		save_current_sctx = current_sctx;
 		current_sctx.sc_sid = SID_MODELINE;
+		current_sctx.sc_seq = 0;
 		current_sctx.sc_lnum = 0;
 #endif
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);