patch 8.2.4603: sourcing buffer lines is too complicated

Problem:    Sourcing buffer lines is too complicated.
Solution:   Simplify the code. Make it possible to source Vim9 script lines.
            (Yegappan Lakshmanan, closes #9974)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 463d840..c33dcbc 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2572,7 +2572,7 @@
 #ifdef FEAT_EVAL
     // Set flag that any command was executed, used by ex_vim9script().
     // Not if this was a command that wasn't executed or :endif.
-    if (getline_equal(ea.getline, ea.cookie, getsourceline)
+    if (sourcing_a_script(&ea)
 	    && current_sctx.sc_sid > 0
 	    && ea.cmdidx != CMD_endif
 	    && (cstack->cs_idx < 0