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>

fsmonitor.c

@@ -309,8 +309,10 @@ void refresh_fsmonitor(struct index_state *istate)
 	enum fsmonitor_reason reason = fsm_settings__get_reason(r);
 
 	if (!warn_once && reason > FSMONITOR_REASON_OK) {
+		char *msg = fsm_settings__get_incompatible_msg(r, reason);
 		warn_once = 1;
-		warning("%s", fsm_settings__get_incompatible_msg(r, reason));
+		warning("%s", msg);
+		free(msg);
 	}
 
 	if (fsm_mode <= FSMONITOR_MODE_DISABLED ||