Initializes a rebase operation to rebase the changes in branch
relative to upstream onto another branch. To begin the rebase
process, call git_rebase_next. When you have finished with this
object, call git_rebase_free.
The commit to begin rebasing from, or NULL to rebase all reachable commits
The branch to rebase onto, or NULL to rebase onto the given upstream
Zero on success; -1 on failure.