Introduce git_rebase_abort

Abort an in-progress rebase and move the working directory and
repository back to the ORIG_HEAD state.
Introduce git_rebase_next

`git_rebase_next` will apply the next patch (or cherry-pick)
operation, leaving the results checked out in the index / working
directory so that consumers can resolve any conflicts, as appropriate.
Write conflicts nicely during rebase application
rebase commit part 1