cache-tree.c: remove the_repository references

cache-tree.c: remove the_repository references

This case is more interesting than other boring "remove the_repo"
commits because while we need access to the object database, we cannot
simply use r->index because unpack-trees.c can operate on a temporary
index, not $GIT_DIR/index. Ideally we should be able to pass an object
database to lookup_tree() but that ship has sailed.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>