t0600: fix incomplete prerequisite for a test case

t0600: fix incomplete prerequisite for a test case

The 'symref transaction supports symlinks' test case is guarded by the
`SYMLINK` prerequisite because `core.prefersymlinkrefs = true` requires
symbolic links to be supported.

However, the `preferSymlinkRefs` feature is not supported on Windows,
therefore this test case needs the `MINGW` prerequisite, too.

There's a couple more cases where we set this config key:

  - In a subsequent test in t0600, but there we explicitly set it to
    "false". So this would naturally be supported by Windows.

  - In t7201 we set the value to `yes`, but we never verify that the
    written reference is a symbolic link in the first place. I guess
    that we could rather remove setting the configuration value here, as
    we are about to deprecate support for symrefs via symbolic links in
    the first place. But that's certainly outside of the scope of this
    patch.

  - In t9903 we do the same, but likewise, we don't check whether the
    written file is a symbolic link.

Therefore this seems to be the only instance where the tests actually
need to be adapted.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>