t1305: skip symlink tests that do not apply to Windows

t1305: skip symlink tests that do not apply to Windows

In Git for Windows, the gitdir is canonicalized so that even when the
gitdir is specified via a symbolic link, the `gitdir:` conditional
include will only match the real directory path.

Unfortunately, t1305 codifies a different behavior in two test cases,
which are hereby skipped on Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>