Store references being fetched in a dictionary.
Store references being fetched in a dictionary.

We need to spawn one swarm per sha, but may need to update multiple
branches, so this provides a mapping between sha's and the branches that
point to it. Storing the information from `got` and `swarms` also seems
more orderly.

larryaubstore/GitTorrent,cjb/GitTorrent,eXcomm/GitTorrent,coopsource/GitTorrent,wakermahmud/GitTorrent,cmotc/GitTorrent,therewillbecode/GitTorrent,locusf/GitTorrent,bowlofstew/GitTorrent,studio666/GitTorrent