Write operations should take a SHA argument that identifies their parent

[mtholder]

We are using lazy branch creation (create WIP on first write not on the GET), which seems good.
But the change set should be applied to the parent that corresponds to the version of the NexSON that the curator was looking at when he/she made the edits.
On write, we should:
1 check for WIP for this study, user, parent commit SHA

1. if not found:
   A. checkout parent commit,
   B. create WIP from that commit,
   C. checkout WIP
2. write new content on WIP then commit and return new SHA

The change we need is 2A (instead of checking out master, we should checkout the correct parent).

[mtholder]

@snacktavish and I have this working on the "local-dep" branch