Fix ref_newer() in send-pack.

Fix ref_newer() in send-pack.

When more than two references need to be checked with
ref_newer() function, the second and later calls did not work
correctly.  This was because the later calls found commits
retained by the "struct object" layer that still had smudges
made by earlier calls.

Signed-off-by: Junio C Hamano <junkio@cox.net>