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