Oxidize `gitu`

[Byron]

Required Features

• [x] revparse-single
• [x] revwalk from given OID and head (probably, question is if the traversal algorithm differs in unforeseen ways)
• [x] list references, peel to commit, shorthand, classification, head, local branches
• [x] commit summary/basic message decoding
• [x] find objects and decode them
• status
  • [x] untracked files
  • [x] conflicted, modified, generally index-to-worktree
  • [ ] tree-to-index
  • [ ] provide facilities to make this easy from status information.
• [x] diff tree to tree
  • [x] easy access to line diffs
• [x] diffing with access to each hunk
  • [x] obtain diff bases exactly like Git would (filters, diff-filters)
• remote branch names
  • [x] branch upstream name
• [ ] ahead-behind information
• [x] state (i.e. MERGING, REBASING
  • [ ] advanced rebase state
  • [ ] advanced merge state
  • Note that gitu accesses some Git-files directly, as such functionality also doesn't exist in git2
• [ ] repository mutation
  • [ ] add and remove files to index (stage, unstage)
  • [x] delete and create branches
  • [ ] rebase
  • [ ] reset & checkout

The corresponding issue in the gitu repository is here: https://github.com/altsem/gitu/issues/12