localrepo: add unbundle support Localrepo now supports the unbundle method of pushing changegroups. We plan to use the unbundle call for bundle2 so it is important that all peers supports it. The `peer.unbundle` and `peer.addchangegroup` code path have small difference so cause some test output changes. None of those changes seems problematic.

unbundle: extract the core logic in another function The `exchange` module now contains an `unbundle` function that holds the core unbundle logic. The wire protocol keeps its own unbundle function. It enforces wireprotocol-specific logic and then calls the extracted function. This aims at implementing unbundle for localrepo.

unbundle: extract checkheads in its own function We are going to refactor the unbundle function to have it working on a local repository too. Having this function extracted will ease the process. In the case of non-matching heads, the function now directly raises an exception. The top level of the function is catching it.