exchange: drop useless line break

subrepo: check return code for git push (issue4223)

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.