Merge branch 'ds/cache-tree-basics'

Document, clean-up and optimize the code around the cache-tree extension in the index. * ds/cache-tree-basics: cache-tree: speed up consecutive path comparisons cache-tree: use ce_namelen() instead of strlen() index-format: discuss recursion of cache-tree better index-format: update preamble to cache tree extension index-format: use 'cache tree' over 'cached tree' cache-tree: trace regions for prime_cache_tree cache-tree: trace regions for I/O cache-tree: use trace2 in cache_tree_update() unpack-trees: add trace2 regions tree-walk: report recursion counts