Code review comment for lp:~qbzr-dev/qbzr/commit_data

I've finished this work. Now this branch supports [commit_data] section in branch.conf with following data:

* message
* bugs
* old_revid - old tip (uncommitted revision)
* new_revid - new tip (after uncommit)

[old_revid, new_revid) -- is uncommitted chain.

Code for support commit_data has been reworked to be as much universal as possible, so in the future it could be reused for other clients. Code for decoding bug urls back to bug numbers or bug ids extracted to separate bugs.py module and also can be reused by other clients. There is enough unittest IMO, but of course there is might be room for improvements.

It works fine in my manual testing. Actually I'm starting to dogfood it.

I'll wait couple of days for review and then planning to land it if nobody will object.