localrepo: extract stream clone application into reusable function The existing stream_in method assumes a streaming clone is applied via the wire protocol. Previous patches have enabled streaming clone data to be produced and consumed outside the context of the wire protocol. However, the consuming part was incomplete because it didn't deal with things like updating the branch caches or writing out a requirements file. This patch finishes the separation of stream clone handling from the wire protocol. After this patch, it is possible to consume stream clones from arbitrary sources, including files. Mozilla plans to leverage this to serve pre-generated stream clone files to consumers, drastically reducing the wall and CPU time required to clone large repositories. This will enable clones to be nearly as fast as `tar`.