bundle2: load hookargs from bundleoperation into transaction when started one
When a transaction is started, we must load the hookargs from the
bundleoperation object to the transaction so that they can be used in the
transaction. Also this patch makes sure no more hookargs are added to the
bundleoperation object once the transaction starts.
This is a part of porting fb extension bundle2hooks to core.
Differential Revision: https://phab.mercurial-scm.org/D209