Problem: Cannot use a script-local function for 'foldtext'.
Solution: Expand "s:" and "<SID>". (Yegappan Lakshmanan, closesvim/vim#9411)
27708e6c7b
Only port the changes actually related to 'foldtext'.
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>