Historically, Gerrit was generous when computing tag reachability and
fed all refs into the computation. For Gerrit instances that have a lot
of changes, this is extremely expensive.
This commit changes that behavior and documents it. The motivation is
that every ref that Gerrit manages (changes, meta refs, etc.) is
Gerrit's domain and there is no use case for users to add tags.
For now, orphaned tags are still served to users who have READ on refs/*
and no block rules configured, but that might as well change in the
future.
Change-Id: I4820ea69fc4c90127a9a5615ae63fabb7dd32175
a9c11a72be