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/digraph.c b/src/digraph.c
index a26d10c..5b56095 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2378,7 +2378,7 @@
      */
     for (;;)
     {
-	line = eap->getline(0, eap->cookie, 0);
+	line = eap->getline(0, eap->cookie, 0, TRUE);
 	if (line == NULL)
 	    break;