move: record both source and destination as predecessors

This matches what we do for `jj squash`, whether it's a
full or partial move.

I didn't add a test since we're planning to deprecate `jj move`, and
this will soon be tested via the `jj squash` tests.

cli/src/commands/squash.rs

@@ -220,6 +220,7 @@ from the source will be moved into the destination.
     tx.mut_repo()
         .rewrite_commit(settings, &destination)
         .set_tree_id(new_destination_tree.id().clone())
+        .set_predecessors(vec![destination.id().clone(), source.id().clone()])
         .set_description(description)
         .write()?;
     Ok(())