PackWriter: Fix the way delta chain cycles are prevented

PackWriter: Fix the way delta chain cycles are prevented

Take a very simple approach to avoiding delta chains during object
reuse: objects are now always selected from the oldest pack that
contains them.  This prevents cycles because a pack must not have
a cycle in the delta chain.  If both objects A and B are chosen
out of the same source pack then there cannot be an A->B->A cycle.

The oldest pack is also the most likely to have the smallest deltas.
Its the biggest pack in the system and probably came from the clone
(or last GC) of this repository, where all objects were previously
considered and packed tightly together.  If an object appears again
(for example due to a revert and a push into this repository) the
newer copy of won't be nearly as small as the older delta version
of it, even if the newer one is also itself a delta.

ObjectDirectory already enumerates objects during selection in this
newest->oldest order, so it already is supplying these assumptions
to PackWriter.  Taking advantage of this can speed up selection by
a tiny amount by avoiding some tests, but can also help to prevent
a cycle needing to be broken on the fly during writing.

The previous cycle breaking logic wasn't fully correct either.
If a different delta base was chosen, the new delta base might not
have been written into the output pack before the current object,
forcing the use of REF_DELTA when OFS_DELTA is always smaller.
This logic has now been reworked to always re-check the delta base
and ensure it gets written before the current object.

If a cycle occurs, it gets broken the same way as before, by
disabling delta reuse and finding an alternative form of the
object, which may require inflating/deflating in whole format.

Change-Id: I9953ab8be54ceb8b588e1280d6f7edd688887747
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>