bundle2: introduce `replycaps` part for on-demand reply The bundle2 processing does not create a bundle2 reply by default anymore. It is only done if the client requests it with a `replycaps` part. This part is called `replycaps` as it will eventually contain data about which bundle2 capabilities are supported by the client. We have to add a flag to the test command to control whether a reply is generated or not.

bundle2: use an official iterparts method to unbundle parts Explicit is better than implicit.

merge: with merge.preferancestor=*, run an auction with bids from ancestors The basic idea is to do the merge planning with all the available ancestors, consider the resulting actions as "bids", make an "auction" and automatically pick the most favourable action for each file. This implements the basic functionality and will only consider "keep" and "get" actions. The heuristics for picking the best action can be tweaked later on. By default it will only pass ctx.ancestor as the single ancestor to calculateupdates. The code path for merging with a single ancestor is not changed.