Document update for nd/unpack-trees-with-cache-tree

Fix an incorrect comment in the new code added in b4da373
(unpack-trees: optimize walking same trees with cache-tree -
2018-08-18) and document about the new test variable that is enabled
by default in test-lib.sh in 4592e60 (cache-tree: verify valid
cache-tree in the test suite - 2018-08-18)

Signed-off-by: Nguyễn Thái Ngọc Duy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pclouds

t/README

@@ -319,6 +319,10 @@ GIT_TEST_OE_DELTA_SIZE=<n> exercises the uncomon pack-objects code
 path where deltas larger than this limit require extra memory
 allocation for bookkeeping.
 
+GIT_TEST_VALIDATE_INDEX_CACHE_ENTRIES=<boolean> checks that cache-tree
+records are valid when the index is written out or after a merge. This
+is mostly to catch missing invalidation. Default is true.
+
 Naming Tests
 ------------
 

unpack-trees.c

@@ -680,8 +680,8 @@ static int index_pos_by_traverse_info(struct name_entry *names,
 
 /*
  * Fast path if we detect that all trees are the same as cache-tree at this
- * path. We'll walk these trees recursively using cache-tree/index instead of
- * ODB since already know what these trees contain.
+ * path. We'll walk these trees in an iterative loop using cache-tree/index
+ * instead of ODB since we already know what these trees contain.
  */
 static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
 				  struct name_entry *names,