Similar to change refs, cache-automerge refs are not tag-able since
they are simply used for Gerrit-internal data representation. Excluding
them from the list of interesting refs in TagSet scales down on the
amount of work we have to do later on when calculating tags.
The fewer references we have prior to calculating the tags, the less
memory we'll need when using BitSets to deal with refs flags.
Change-Id: I25b301bba5f22e94c6339a6bf2e3b87a505202f3
f9e3ef0b53