AmazonS3: Speed up fetch, try recent packs first

When fetching remote objects, WalkFetchConnection searches remote
packs in the order provided by WalkRemoteObjectDatabase:getPackNames.
Previously, for TransportAmazonS3, the packs were in no particular
order. This resulted in potential many extra calls to get pack idx
files.

This change modifies TransportAmazonS3 and AmazonS3 so that
getPackNames returns a list sorted with the most recently modified
packs first.  In the case of fetching recent changes to a repo,
this dramatically reduces the number of packs searched and speeds
up fetch.

Note: WalkRemoteObjectDatabase::getPackNames does not specify
the order of the returned names.

Testing: did "mvn clean install" in root dir and all tests passed.
And manually constructed some S3 repos and using jgit.sh
confirmed that the freshest pack was checked first.

Change-Id: I3b968fee825e793be55566e28c2d69d0cbe53807
Signed-off-by: Joshua Redstone <redstone@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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