This works in a similar manner to git-check-attr.

Thanks to Jeff King and Junio C Hamano for the idea:
http://thread.gmane.org/gmane.comp.version-control.git/108671/focus=108815Signed-off-by: NAdam Spiers <git@adamspiers.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>