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

> When I type C-x C-f xmail/foox TAB, where xmail contains 800 files
> and none of them starts with foox, it takes a few seconds (including
> 3 GCs) before telling me it can't be found.
> Here's output from xbacktrace:

>     "file-name-completion" (0x7fbe23e4)

Indeed, the new partial-completion code triggers an inefficiency in
file-name-completion.  I've installed a patch which should significantly
reduce the problem (at least in my test case, tho I had to use
a directory with 100K files to reproduce a problem that seems similar to
yours).


        Stefan