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

[Richard M Stallman <rms@gnu.org>]

    Can you try the patch below to confirm it fixes your problem?

The problem in my real cases is fixed given the sources as of
yesterday.  It looks like you've identified fixes to other big
slowdowns that arise in yet larger test cases.  I don't need them,
but now that you've written them, you may as well install them
unless they will cause trouble for someone.

    In the case of file completion, a simple (file-directory-p "xmail/")
    would give us the necessary answer, but the code is generic and the
    only/best way to get this information from a completion tables is to
    call (try-completion "xmail/" table), which calls (file-name-completion
    "" "xmail/").

I have a feeling that (file-name-completion "" "xmail/") ought to
first check (file-directory-p "xmail/") and return t if that does.  In
other words, if the buffer contains a valid directory name, completion
should treat that as fixed.  It should only try to do completion on
previous file name components when they don't match existing names.