pack-objects: allow "thin" packs to exceed depth limits

pack-objects: allow "thin" packs to exceed depth limits

When creating a new pack to be used in .git/objects/pack/
directory, we carefully count the depth of deltified objects to
be reused, so that the generated pack does not to exceed the
specified depth limit for runtime efficiency.  However, when we
are generating a thin pack that does not contain base objects,
such a pack can only be used during network transfer that is
expanded on the other end upon reception, so being careful and
artificially cutting the delta chain does not buy us anything
except increased bandwidth requirement.  This patch disables the
delta chain depth limit check when reusing an existing delta.

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