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
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