tree.c: make read_tree*() take 'struct repository *'

These functions call tree_entry_interesting() which will soon require
a 'struct index_state *' to be passed in. Instead of just changing the
function signature to take an index, update to take a repo instead
because these functions do need object database access.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>