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

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