bundle2: move part processing to a separate function Now that the part processing loop is tiny, let's move it to a separate function. This will allow extensions to completely replace the part processing logic, without having to replace the overall bundle processing logic or the stream maintenance logic. This will be useful for the infinitepush extension, so it can completely take over receiving a bundle and rerouting it to a side store. This will also make it easier to upstream the infinitepush functionality later. Differential Revision: https://phab.mercurial-scm.org/D709