rebase: allow rebasing onto different base.

rebase: allow rebasing onto different base.

This allows you to rewrite history a bit more flexibly, by
separating the other branch name and new branch point.  By
default, the new branch point is the same as the tip of the
other branch as before, but you can specify where you graft the
rebased branch onto.

When you have this ancestry graph:

          A---B---C topic
         /
    D---E---F---G master

	$ git rebase --onto master~1 master topic

would rewrite the history to look like this:

	      A'\''--B'\''--C'\'' topic
	     /
    D---E---F---G master

Signed-off-by: Junio C Hamano <junkio@cox.net>