Handle point in last file-name component in minibuffer completion

Handle point in last file-name component in minibuffer completion

This is a followup to commit e338a8ac41d4a9fd798dda90275abe75ac071335
(Handle point not at EOB in minibuffer-choose-completion).
That commit added a heuristic, but the heuristic was insufficient:
It still had the original wrong behavior when completing the last
file-name component (i.e., the completion category is 'file' and
there's no slash after point).  This patch makes the heuristic
cover that case as well.
* lisp/minibuffer.el (minibuffer-next-completion)
(minibuffer-choose-completion): If in file completion and there's no
slash after point, clear what's after point when we complete.
(Bug#62700)