Talk about <sha1-file.c> and <object.h> family, things like
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read_sha1_file() 
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read_object_with_reference() 
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has_sha1_file() 
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write_sha1_file() 
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pretend_object_file() 
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lookup_{object,commit,tag,blob,tree} 
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parse_{object,commit,tag,blob,tree} 
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Use of object flags 
(JC, Shawn, Daniel, Dscho, Linus)