add pretxncommit hook. hook allows check of changeset after create, but before transaction is committed. hook failure rolls transaction back. makes place for local policies like commit message must contain bug id or reviewer signoff. change also adds parent changeset ids to commit hook environment, because is cheap and useful.

add pretag and tag hooks. pretag hook lets hook decide how tags can be named. tag hook is notifier.

hook parameters must be strings.

make hook code nicer. pre hooks can raise exception if fail. tidy up better if exception raised (like KeyboardInterrupt) when running hook program.