repo/private: allow committing/aborting through a transaction guard

This enhances the auto-transaction logic, augmenting the scope of a
transaction guard.
It allows committing or aborting a transaction through its guard.
It also supports tracking the completion status of a transaction
guard, avoiding double commits/aborts, while retaining the auto-cleanup
logic.

src/libostree/ostree-sysroot-cleanup.c

@@ -445,7 +445,7 @@ generate_deployment_refs (OstreeSysroot       *self,
                            cancellable, error))
     return FALSE;
 
-  g_autoptr(_OstreeRepoAutoTransaction) txn =
+  g_autoptr(OstreeRepoAutoTransaction) txn =
     _ostree_repo_auto_transaction_start (repo, cancellable, error);
   if (!txn)
     return FALSE;
@@ -458,7 +458,7 @@ generate_deployment_refs (OstreeSysroot       *self,
 
       ostree_repo_transaction_set_refspec (repo, refname, ostree_deployment_get_csum (deployment));
     }
-  if (!ostree_repo_commit_transaction (repo, NULL, cancellable, error))
+  if (!_ostree_repo_auto_transaction_commit (txn, NULL, cancellable, error))
     return FALSE;
 
   return TRUE;