fsck: accept an oid instead of a "struct tree" for fsck_tree()

fsck: accept an oid instead of a "struct tree" for fsck_tree()

We don't actually look at the tree struct in fsck_tree() beyond its oid
and type (which is obviously OBJ_TREE). Just taking an oid gives our
callers more flexibility to avoid creating a struct, and makes it clear
that we are fscking just what is in the buffer, not any pre-parsed bits
from the struct.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>