Merge branch 'jt/transfer-fsck-with-promissor'

The transfer.fsckobjects configuration tells "git fetch" to validate the data and connected-ness of objects in the received pack; the code to perform this check has been taught about the narrow clone's convention that missing objects that are reachable from objects in a pack that came from a promissor remote is OK. * jt/transfer-fsck-with-promissor: fetch-pack: do not check links for partial fetch index-pack: support checking objects but not links