fsmonitor: fix leak of warning message

fsmonitor: fix leak of warning message

The fsm_settings__get_incompatible_msg() function returns an allocated
string.  So we can't pass its result directly to warning(); we must hold
on to the pointer and free it to avoid a leak.

The leak here is small and fixed size, but Coverity complained, and
presumably SANITIZE=leaks would eventually.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>