wildmatch test: create & test files on disk in addition to in-memory

wildmatch test: create & test files on disk in addition to in-memory

There has never been any full roundtrip testing of what git-ls-files
and other commands that use wildmatch() actually do, rather we've been
satisfied with just testing the underlying C function.

Due to git-ls-files and friends having their own codepaths before they
call wildmatch() there's sometimes differences in the behavior between
the two. Even when we test for those (as with [1]), there was no one
place where you can review how these two modes differ.

Now there is. We now attempt to create a file called $haystack and
match $needle against it for each pair of $needle and $haystack that
we were passing to test-wildmatch.

If we can't create the file we skip the test. This ensures that we can
run this on all platforms and not maintain some infinitely growing
whitelist of e.g. platforms that don't support certain characters in
filenames.

A notable exception to this is Windows, where due to the reasons
explained in [2] the shellscript emulation layer might fake the
creation of a file such as "*", and "test -e" for it will succeed
since it just got created with some character that maps to "*", but
git ls-files won't be fooled by this.

Thus we need to skip creating certain filenames entirely on Windows,
the list here might be overly aggressive. I don't have access to a
Windows system to test this.

As a result of doing these tests we can now see the cases where these
two ways of testing wildmatch differ:

 * Creating a file called 'a[]b' and running ls-files 'a[]b' will show
   that file, but wildmatch("a[]b", "a[]b") will not match

 * wildmatch() won't match a file called \ against \, but ls-files
   will.

 * `git --glob-pathspecs ls-files 'foo**'` will match a file
   'foo/bba/arr', but wildmatch won't, however pathmatch will.

   This seems like a bug to me, the two are otherwise equivalent as
   these tests show.

This also reveals the case discussed in [1], since 2.16.0 '' is now an
error as far as ls-files is concerned, but wildmatch() itself happily
accepts it.

1. 9e4e8a64c2 ("pathspec: die on empty strings as pathspec",
   2017-06-06)

2. nycvar.QRO.7.76.6.1801052133380.1337@wbunaarf-fpuvaqryva.tvgsbejvaqbjf.bet
   (https://public-inbox.org/git/?q=nycvar.QRO.7.76.6.1801052133380.1337%40wbunaarf-fpuvaqryva.tvgsbejvaqbjf.bet)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>