Merge branch 'pk/rebase-in-c-2-basic'

Rewrite "git rebase" in C.

* pk/rebase-in-c-2-basic:
  builtin rebase: support `git rebase <upstream> <switch-to>`
  builtin rebase: only store fully-qualified refs in `options.head_name`
  builtin rebase: start a new rebase only if none is in progress
  builtin rebase: support --force-rebase
  builtin rebase: try to fast forward when possible
  builtin rebase: require a clean worktree
  builtin rebase: support the `verbose` and `diffstat` options
  builtin rebase: support --quiet
  builtin rebase: handle the pre-rebase hook and --no-verify
  builtin rebase: support `git rebase --onto A...B`
  builtin rebase: support --onto

Author: gitster