Add/document commit-hook

[Marcus Lindblom <macke@…>]

There is a commit hook here which IMHO should be incorporated into the plugin.

The comments also refer to a built-in hook, but I was unable to find it (neither in code nor on the TracMercurial page).

[cboos]

The hook itself: http://www.bitbucket.org/madssj/mercurial-trac-hook/src/tip/trachook.py

Surely having a changeset hook for TracMercurial would be a good thing, but I prefer to base that on the new emerging interface for changeset change notification, as started in #7723.

That way the mercurial hook itself will be quite minimalistic, and the actions to be taken (closing tickets, etc.) would be done externally.