Convert the sha1 member of struct cache_tree to struct object_id by
changing the definition and applying the following semantic patch, plus
the standard object_id transforms:
Fix up one reference to active_cache_tree which was not automatically
caught by Coccinelle. These changes are prerequisites for converting
parse_object.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>