hooks: add HG_NODE_LAST to txnclose and changegroup hook environments Sometimes a txnclose or changegroup hook wants to iterate through all the changesets in transaction: in that situation usually the revset `$HG_NODE:` is used to select the revisions. Unfortunately this revset sometimes may contain too many changesets because we don't have the write lock while the hook runs newer changes may be added to repository in the meantime. That's why there is a need for extra variable carrying the information about the last change in the transaction.