Don't load missing ACL files in paranoid update hook

Don't load missing ACL files in paranoid update hook

If a user or group ACL file does not exist in the current tip
revision of the acl repository we will get an error from cat-file
when we ask for that blob as it cannot be resolved.  A quick look
at the history by rev-list can tell us if there is a path there
or not.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>