However, the draft stage isn't documented this way. Members negotiate whatever between themselves (well, really their staffers) and this happens over email, in discussions, via Word documents - whatever works.

I guess in the git metaphor, drafts are in flux while being worked on as a commit, and are squashed and then accessible as such squashed commits once initially introduced or whenever they lead to bill amendments. You can't necessarily track down what member was responsible for a specific sentence in an amendment.