bundle-uri: download in creationToken order

bundle-uri: download in creationToken order

The creationToken heuristic provides an ordering on the bundles
advertised by a bundle list. Teach the Git client to download bundles
differently when this heuristic is advertised.

The bundles in the list are sorted by their advertised creationToken
values, then downloaded in decreasing order. This avoids the previous
strategy of downloading bundles in an arbitrary order and attempting
to apply them (likely failing in the case of required commits) until
discovering the order through attempted unbundling.

During a fresh 'git clone', it may make sense to download the bundles in
increasing order, since that would prevent the need to attempt
unbundling a bundle with required commits that do not exist in our empty
object store. The cost of testing an unbundle is quite low, and instead
the chosen order is optimizing for a future bundle download during a
'git fetch' operation with a non-empty object store.

Since the Git client continues fetching from the Git remote after
downloading and unbundling bundles, the client's object store can be
ahead of the bundle provider's object store. The next time it attempts
to download from the bundle list, it makes most sense to download only
the most-recent bundles until all tips successfully unbundle. The
strategy implemented here provides that short-circuit where the client
downloads a minimal set of bundles.

However, we are not satisfied by the naive approach of downloading
bundles until one successfully unbundles, expecting the earlier bundles
to successfully unbundle now. The example repository in t5558
demonstrates this well:

 ---------------- bundle-4

       4
      / \
 ----|---|------- bundle-3
     |   |
     |   3
     |   |
 ----|---|------- bundle-2
     |   |
     2   |
     |   |
 ----|---|------- bundle-1
      \ /
       1
       |
 (previous commits)

In this repository, if we already have the objects for bundle-1 and then
try to fetch from this list, the naive approach will fail. bundle-4
requires both bundle-3 and bundle-2, though bundle-3 will successfully
unbundle without bundle-2. Thus, the algorithm needs to keep this in
mind.

A later implementation detail will store the maximum creationToken seen
during such a bundle download, and the client will avoid downloading a
bundle unless its creationToken is strictly greater than that stored
value. For now, if the client seeks to download from an identical
bundle list since its previous download, it will download the
most-recent bundle then stop since its required commits are already in
the object store.

Add tests that exercise this behavior, but we will expand upon these
tests when incremental downloads during 'git fetch' make use of
creationToken values.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>