follow_library_folder rebuilt the chosen subfolder from the lower-cased
folder_key on Windows and joined it with '/', so after a move the folder
shown in the sidebar came back as c:\archive\trip/day1 with its names in
lower case. Add folder_rest (compares by folder_key, returns the names as
spelled) and join them with the platform's separator. Tests compare
folders by identity where they only mean the same folder, and a new test
fails on Windows without the fix.