bookmarks: fix _bookmarks/lookup() reentrancy issue (issue2016) _bookmarks is loaded lazily and calls super.lookup(). Unfortunately, branch and tags caches initializations also recurse in lookup() and end up trying to access _bookmarks again. Massive confusion ensues. I considered fixing all branches and tags cache loading to avoid recursing in lookup() but it would add complexity to otherwise working code provided lookups are performed on nodes or revnums.