Merge branch 'vd/skip-cache-tree-update'

Avoid calling 'cache_tree_update()' when doing so would be redundant. * vd/skip-cache-tree-update: rebase: use 'skip_cache_tree_update' option read-tree: use 'skip_cache_tree_update' option reset: use 'skip_cache_tree_update' option unpack-trees: add 'skip_cache_tree_update' option cache-tree: add perf test comparing update and prime