patch 8.2.4594: need to write script to a file to be able to source them

Problem:    Need to write script to a file to be able to source them.
Solution:   Make ":source" use lines from the current buffer. (Yegappan
            Lakshmanan et al., closes #9967)
diff --git a/src/eval.c b/src/eval.c
index 8d1e4fd..a963a15 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -140,7 +140,7 @@
     if (eap != NULL)
     {
 	evalarg->eval_cstack = eap->cstack;
-	if (getline_equal(eap->getline, eap->cookie, getsourceline))
+	if (sourcing_a_script(eap))
 	{
 	    evalarg->eval_getline = eap->getline;
 	    evalarg->eval_cookie = eap->cookie;