In some applications of this paranoid update hook the set of ACL
rules that need to be applied to a user can be large, and the
number of users that those rules must also be applied to can be
more than a handful of individuals.  Rather than repeating the same
rules multiple times (once for each user) we now allow users to be
members of groups, where the group supplies the list of ACL rules.
For various reasons we don't depend on the underlying OS groups
and instead perform our own group handling.

Users can be made a member of one or more groups by setting the
user.memberOf property within the "users/$who.acl" file:

  [user]
    memberOf = developer
	memberOf = administrator

This will cause the hook to also parse the "groups/$groupname.acl"
file for each value of user.memberOf, and merge any allow rules
that match the current repository with the user's own private rules
(if they had any).

Since some rules are basically the same but may have a component
differ based on the individual user, any user.* key may be inserted
into a rule using the "${user.foo}" syntax.  The allow rule does
not match if the user does not define one (and exactly one) value
for the key "foo".

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