Talk about <read-cache.c> and <cache-tree.c>, things like:
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cache → the_index macros 
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read_index() 
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write_index() 
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ie_match_stat() and ie_modified(); how they are different and when to use which. 
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index_name_pos() 
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remove_index_entry_at() 
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remove_file_from_index() 
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add_file_to_index() 
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add_index_entry() 
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refresh_index() 
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discard_index() 
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cache_tree_invalidate_path() 
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cache_tree_update() 
(JC, Linus)