Oxidize `gitu`

[Byron]

Required Features

• revparse-single
• revwalk from given OID and head (probably, question is if the traversal algorithm differs in unforeseen ways)
• list references, peel to commit, shorthand, classification, head, local branches
• commit summary/basic message decoding
• find objects and decode them
• status
  • untracked files
  • conflicted, modified, generally index-to-worktree
  • tree-to-index
  • provide facilities to make this easy from status information.
• diff tree to tree
  • easy access to line diffs
• diffing with access to each hunk
  • obtain diff bases exactly like Git would (filters, diff-filters)
• remote branch names
  • branch upstream name
• ahead-behind information
• 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)
  • delete and create branches
  • rebase
  • reset & checkout

The corresponding issue in the gitu repository is here: altsem/gitu#12