localrepo: pass hook argument txnid to pretxnopen hooks
Before this patch, hook argument `txnid` isn't passed to `pretxnopen`
hooks, even though `hooks` section of `hg help config` describes so.
``pretxnopen``
Run before any new repository transaction is open. The reason for the
transaction will be in ``$HG_TXNNAME`` and a unique identifier for the
transaction will be in ``HG_TXNID``. A non-zero status will prevent the
transaction from being opened.