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>

Author: dscho

t/t1305-config-include.sh

@@ -286,7 +286,7 @@ test_expect_success SYMLINKS 'conditional include, relative path with symlinks'
 	)
 '
 
-test_expect_success SYMLINKS 'conditional include, gitdir matching symlink' '
+test_expect_success SYMLINKS,!MINGW 'conditional include, gitdir matching symlink' '
 	ln -s foo bar &&
 	(
 		cd bar &&
@@ -298,7 +298,7 @@ test_expect_success SYMLINKS 'conditional include, gitdir matching symlink' '
 	)
 '
 
-test_expect_success SYMLINKS 'conditional include, gitdir matching symlink, icase' '
+test_expect_success SYMLINKS,!MINGW 'conditional include, gitdir matching symlink, icase' '
 	(
 		cd bar &&
 		echo "[includeIf \"gitdir/i:BAR/\"]path=bar8" >>.git/config &&