git-p4: skip t9819 test case on case insensitive file systems

Windows and OS X file systems are case insensitive by default.
Consequently the "git-p4-case-folding" test case does not apply to
them.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9819-git-p4-case-folding.sh

@@ -4,6 +4,12 @@ test_description='interaction with P4 case-folding'
 
 . ./lib-git-p4.sh
 
+if test_have_prereq CASE_INSENSITIVE_FS
+then
+	skip_all='skipping P4 case-folding tests; case insensitive file system detected'
+	test_done
+fi
+
 test_expect_success 'start p4d with case folding enabled' '
 	start_p4d -C1
 '