contint: tweak Zuul git apache rule

We get to apply the Order directive on the ScriptAlias destination, and
specially to the directory holding the script: /usr/lib/git-core

Also changed the Order rule to Allow,Deny, which would deny anything
uneless the request is solely matched by an Allow.

Change-Id: Ibbcc127e624e596a32ee94d0861acd9491461e69