Merge branch 'pk/rebase-in-c-3-acts'

Rewrite "git rebase" in C. * pk/rebase-in-c-3-acts: builtin rebase: stop if `git am` is in progress builtin rebase: actions require a rebase in progress builtin rebase: support --edit-todo and --show-current-patch builtin rebase: support --quit builtin rebase: support --abort builtin rebase: support --skip builtin rebase: support --continue