Problem: using f-flag in 'complete' conflicts with Neovims filename
completion (glepnir, after v9.1.1301).
Solution: use upper-case "F" flag for completion functions
(Girish Palya).
fixes: vim/vim#17347closes: vim/vim#1737814f6da5ba8
Co-authored-by: Girish Palya <girishji@gmail.com>