rebase_init
Description:
public Rebase? rebase_init (AnnotatedCommit? branch, AnnotatedCommit? upstream, AnnotatedCommit? onto, RebaseOptions options) throws Error
  
  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 
      unref.
Parameters:
| this | a Repository. | 
| branch | the terminal commit to rebase, or null to rebase the current branch. | 
| upstream | the commit to begin rebasing from, or null to rebase all reachable commits. | 
| onto | the branch to rebase onto, or null to rebase onto the given upstream. | 
| options | a RebaseOptions to specify how rebase is performed, or null. | 
Returns:
| a newly allocated Rebase. |