transaction: introduce "changes" dictionary to precisely track updates The transaction is already tracking some data intended for hooks (in 'hookargs'). However, that information is minimal as we optimise for passing data to other processes through environment variables. There are multiple places were we could use more complete and lower level information locally (eg: cache update, better report of changes to hooks, etc...). For this purpose we introduces a 'changes' dictionary on the transaction. It is intended to track every changes happening to the repository (eg: new revs, bookmarks move, phases move, obs-markers, etc). For now we just adds the 'changes' dictionary. We'll adds more tracking and usages over time.