vim-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.
ded5f1bed7

src/nvim/buffer.c

@@ -5155,6 +5155,7 @@ chk_modeline(
       const int secure_save = secure;
       const sctx_T save_current_sctx = current_sctx;
       current_sctx.sc_sid = SID_MODELINE;
+      current_sctx.sc_seq = 0;
       current_sctx.sc_lnum = 0;
       // Make sure no risky things are executed as a side effect.
       secure = 1;