The setup of alternates has been deferred to object database
creation time during clone, which drops the unused ad-hoc alternate
writing API, simplifying the object database backend interface.
* ps/odb-alternates-at-creation:
odb/source: remove the ability to write alternates
builtin/clone: write alternates via `odb_create_on_disk()`
odb/source: support writing alternates when creating the database
builtin/clone: move setup of alternates for non-shared local clones
builtin/clone: move setup of alternates for shared local clones
builtin/clone: refactor handling of "--reference{,-if-able}"
builtin/clone: move around `setup_reference()`
builtin/clone: defer setup of the object database
7dfd47023a