Upstream git.git has changed the way how packfiles are named.
Previously, they were using a hash of the contained object's OIDs, which
has then been changed to use the hash of the complete packfile instead.
See 1190a1acf (pack-objects: name pack files after trailer hash,
2013-12-05) in the git.git repository for more information on this
change.

This commit changes our logic to match the behavior of core git.