Merge branch 'ps/connected-generic-promisor-checks'

Merge branch 'ps/connected-generic-promisor-checks'

The connectivity check has been refactored to search for promisor
objects in a generic way using the object database interface,
rather than iterating packfiles directly. This allows connectivity
checks to work properly in repositories that do not use packfiles.

* ps/connected-generic-promisor-checks:
  connected: search promisor objects generically
  connected: split out promisor-based connectivity check
  odb/source-packed: support flags when iterating an object prefix
  odb/source-packed: extract logic to skip certain packs