When appending objects to a pack, make sure the appended data is really
what we expect instead of simply loading potentially corrupted objects
and legitimating them by computing a SHA1 of that corrupt data.

With this the sha1_object() can lose its test_for_collision parameter
which is now redundent.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>