t0050: honor CASE_INSENSITIVE_FS in add (with different case)

The test case "add (with different case)" indicates a
known breakage when run on a case insensitive file system.

The test is invalid for case sensitive file system, it will always fail.

Check the precondition CASE_INSENSITIVE_FS before running it.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0050-filesystem.sh

@@ -88,7 +88,7 @@ test_expect_success 'merge (case change)' '
 
 
 
-test_expect_failure 'add (with different case)' '
+test_expect_failure CASE_INSENSITIVE_FS 'add (with different case)' '
 
 	git reset --hard initial &&
 	rm camelcase &&