patch 8.2.3146: Vim9: line number wrong for :execute argument

Problem:    Vim9: line number wrong for :execute argument.
Solution:   Use the line number of the :execute command itself. (closes #8537)
diff --git a/src/eval.c b/src/eval.c
index 6537ffe..5dc1642 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -6191,6 +6191,7 @@
     char_u	*p;
     garray_T	ga;
     int		len;
+    long	start_lnum = SOURCING_LNUM;
 
     ga_init2(&ga, 1, 80);
 
@@ -6244,6 +6245,9 @@
 
     if (ret != FAIL && ga.ga_data != NULL)
     {
+	// use the first line of continuation lines for messages
+	SOURCING_LNUM = start_lnum;
+
 	if (eap->cmdidx == CMD_echomsg || eap->cmdidx == CMD_echoerr)
 	{
 	    // Mark the already saved text as finishing the line, so that what