httppeer: move requestbuilder defaults into makepeer() argument Upcoming commits will move the initial ?cmd=capabilities handshake request out of httppeer so the handshake can be performed before a peer instance is constructed. In order to do this, we'll need to refactor code for making HTTP requests. The type used to construct HTTP requests is configurable. If we'll be making HTTP requests outside of httppeer, we should be able to use a custom request builder. So move the definition of that type into makepeer(). Extensions can monkeypatch the function and override the argument value. Differential Revision: https://phab.mercurial-scm.org/D3232