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/ops.c b/src/ops.c
index 826a03c..a0b91f9 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -788,7 +788,7 @@
 {
     char_u	*new_line;
 
-    new_line = getcmdline('=', 0L, 0);
+    new_line = getcmdline('=', 0L, 0, TRUE);
     if (new_line == NULL)
 	return NUL;
     if (*new_line == NUL)	/* use previous line */