mingw32-unzip: Add mingw version of unzip

The available msys version of unzip causes
some tests from t5003-archive-zip.sh from the git
test suite to either hang indefinitly or fail.

I've used 1b38b168aed17937270d29b58ec1fee16df46acb on top of your mingw32-git branch for test execution.

```sh
GIT_UNZIP="/bin/unzip" prove -v t5003-archive-zip.sh
[....]
DIED. FAILED tests 9-11, 17-19, 21-23, 25
        Failed 10/27 tests, 62.96% okay (less 1 skipped test: 16 okay, 59.26%)
Failed Test          Stat Wstat Total Fail  Failed  List of Failed
-------------------------------------------------------------------------------
t5003-archive-zip.sh    1   256    27   10  37.04%  9-11 17-19 21-23 25
1 subtest skipped.
Failed 1/1 test scripts, 0.00% okay. 10/27 subtests failed, 62.96% okay.
```

versus the new mingw-built unzip

```sh
GIT_UNZIP="/mingw/bin/unzip" prove -v t5003-archive-zip.sh
1/27 skipped: add symlink (missing UNZIP_SYMLINKS,SYMLINKS of SYMLINKS,UNZIP_SYMLINKS)
All tests successful, 1 subtest skipped.
```