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/normal.c b/src/normal.c
index 834b231..cf9026f 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -6237,7 +6237,7 @@
 
     /* When using 'incsearch' the cursor may be moved to set a different search
      * start position. */
-    cap->searchbuf = getcmdline(cap->cmdchar, cap->count1, 0);
+    cap->searchbuf = getcmdline(cap->cmdchar, cap->count1, 0, TRUE);
 
     if (cap->searchbuf == NULL)
     {