merge: consistently use single quotes for non-user-facing strings Because I'm getting tired of searching for both 'O' and "O".

localrepo: add a currenttransaction method This method returnx the current transaction or None: it will allow a cache writer to hook in an existing transaction.

repoview: extract actual hidden cache writing in its own function This will allow the generation of this cache within the transaction. Relying on the transaction will reduce the chance of reader seeing bad cache.