t2019: skip test requiring '*' in a file name non Windows

A test case introduced by ae454f61 (Add tests for wildcard "path vs ref"
disambiguation) allocates a file named '*.c'. This does not work on
Windows, because the OS forbids file names containing wildcard
characters. The test case fails where the shell attempts to allocate the
file. Skip the test on Windows.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2019-checkout-ambiguous-ref.sh

@@ -69,7 +69,7 @@ test_expect_success 'wildcard ambiguation, paths win' '
 	)
 '
 
-test_expect_success 'wildcard ambiguation, refs lose' '
+test_expect_success !MINGW 'wildcard ambiguation, refs lose' '
 	git init ambi2 &&
 	(
 		cd ambi2 &&