Code review comment for lp:~ian-clatworthy/bzr/finish_commit_hook_2

This patch is required for content filtering to Do The Right Thing. In particular, it provides a hook so that bzr-keywords can update the working tree and expand keywords in files just committed. See bug 385879.

This patch supersedes https://code.edge.launchpad.net/~ian-clatworthy/bzr/finish-commit-hook/+merge/10973 and introduces a FinishCommitHookParams class as discussed with jam during the review of that earlier branch.