patch 8.1.1588: in :let-heredoc line continuation is recognized

Problem:    In :let-heredoc line continuation is recognized.
Solution:   Do not consume line continuation. (Ozaki Kiichi, closes #4580)
diff --git a/src/eval.c b/src/eval.c
index ac9930b..f622b6f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1307,7 +1307,7 @@
 	int	mi = 0;
 	int	ti = 0;
 
-	theline = eap->getline(NUL, eap->cookie, 0);
+	theline = eap->getline(NUL, eap->cookie, 0, FALSE);
 	if (theline == NULL)
 	{
 	    semsg(_("E990: Missing end marker '%s'"), marker);