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/testdir/test_let.vim b/src/testdir/test_let.vim
index 7a547ce..f9c0d0d 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -237,6 +237,14 @@
   END
   call assert_equal(['something', 'endfunc'], var1)
 
+  " not concatenate lines
+  let var1 =<< END
+some
+  \thing
+  \ else
+END
+  call assert_equal(['some', '  \thing', '  \ else'], var1)
+
   " ignore "python << xx"
   let var1 =<<END
 something
diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index 41bb12f..6b678fb 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -157,6 +157,7 @@
 " horizontally or vertically.
 func Test_o_arg()
   let after =<< trim [CODE]
+    set cpo&vim
     call writefile([winnr("$"),
 		\ winheight(1), winheight(2), &lines,
 		\ winwidth(1), winwidth(2), &columns,