commit: Mark ostree_repo_transaction_set_ref* checksums nullable

Allow GI bindings to delete refs through ostree_repo_transaction_set_ref
and ostree_repo_transaction_set_refspec by setting the checksum to NULL.

Closes: #834
Approved by: cgwalters

tests/test-core.js

@@ -52,4 +52,18 @@ let child = root.get_child('some-file');
 let info = child.query_info("standard::name,standard::type,standard::size", 0, null);
 assertEquals(info.get_size(), 12);
 
+// Write a ref and read it back
+repo.prepare_transaction(null);
+repo.transaction_set_refspec('someref', commit);
+repo.commit_transaction(null, null);
+let [,readCommit] = repo.resolve_rev('someref', false);
+assertEquals(readCommit, commit);
+
+// Delete a ref
+repo.prepare_transaction(null);
+repo.transaction_set_refspec('someref', null);
+repo.commit_transaction(null, null);
+[,readCommit] = repo.resolve_rev('someref', true);
+assertEquals(readCommit, null);
+
 print("test-core complete");