This patch adds support to rebase to show the changes in node once the rebase is
complete. This will be extremely helpful for automation purposes and editors
such as Nuclide.
The output is a dictionary of predecessor hash as key and a list of successors'
hashes. The successors one is a list as there can be many successors for a single
predecessor in case of split and it will good to have a generic output format.
This patch adds tests for the same. A new file is created for the patch as
existing files related to rebase has their own purpose and there will be more
formatter support coming for rebase in next cycle.
Thanks to Jun for suggesting to use fm.data().