implement test_might_fail using a refactored test_must_fail

implement test_might_fail using a refactored test_must_fail

Add an (optional) first parameter "ok=<special case>" to test_must_fail
and return success for "<special case>". Add "success" as
"<special case>" and use it to implement "test_might_fail". This removes
redundancies in test-lib-function.sh.

You can pass multiple <special case> arguments divided by comma (e.g.
"test_must_fail ok=success,something")

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Jeff King <peff@peff.net>