bug#2445: 23.0.90; file name completion GCs a lot

[Richard M Stallman]

    > I have a feeling that (file-name-completion "" "xmail/") ought to
    > first check (file-directory-p "xmail/") and return t if that does.

    No: if all files in xmail/ start with "aba", then the above should
    return "aba".

I am not convinced it should do that.  Maybe
(file-name-completion "xmail" "") should do that,
but I tend to think that (file-name-completion "" "xmail/"), with the
empty string as file name, should not complete anything.

There is a semantic confusion going on here.
The question that (try-completion "xmail/" table) answers
is not the question that its caller wants to ask.