acl: use of "!" prefix in user or group names
The "!" prefix allows you to prevent anyone except a given user or group to push
changesets in a given branch or path.
This patch enables a use case suggested by a user (Julien Bonnet):
There's a branch that only a given user (or group) should be able to push to,
and you don't want to restrict access to any other branch that may be created.
With the "!" prefix, you simply deny access to "!givenuser" (or "!@givengroup").