Remove the PBGitSHA class and use the GTOID class from Objective-Git instead.
Remove unnecessary copying/passing by value of git_oid
Provide a "User Cancelled" error so we don't crash when canceling "New".

Closes #275, #220.
Only directories, please.
Make "Clone" an alternate of "New", and wire it up to the document controller.
WIP: Redo repository cloning.