vim-patch:8.1.1588: in :let-heredoc line continuation is recognized (#29767)

Problem:    In :let-heredoc line continuation is recognized.
Solution:   Do not consume line continuation. (Ozaki Kiichi, closes vim/vim#4580)

e96a2498f9

Nvim already sets may_garbage_collect to false in nv_event(), so the
timer change isn't needed.
Other changes have already been ported.
Also fix incorrect port of test in patch 8.1.1356.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_let.vim

@@ -563,6 +563,14 @@ END
   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

test/old/testdir/test_startup.vim

@@ -163,6 +163,7 @@ endfunc
 " 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,