Discard cache-automerge/* refs early in VisibleRefFilter.

By discarding refs in refs/cache-automerge/ early in
VisibleRefFilter, we can make life easier for both
the TagMatcher and later RevWalks.

For typical large Git, with many refs and lots of cached merges,
we could cut the push time down significantly.

Change-Id: I1c1c1bda1944f42bc5a64c02114045e7bbd9bede
1 file changed