update_subtree.sh: add reviewers to downstream patch commit
update_subtree.sh: add reviewers to downstream patch commit

Some subtrees may have downstream changes that are patched on top of the
merged upstream changes. The tool currently only adds reviewers to the
upstream changes. It has been modified to also add reviewer to the
downstream patch commit, if it exists.

Change-Id: I5cd7ffb5077128bbf9dfee43ab8753c664c8633a
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/140556
Tested-by: Jenkins Server <8e3f934e4c44875bc48d33da3ea13d93ba9a233f@us.ibm.com>
Reviewed-by: Benjamen G Tyner <506b6a2e26cca05d0eb1ae91301b5a4a8d39199d@ibm.com>
Reviewed-by: Caleb N Palmer <4dd1938fbd016e66315ec83516544b027a7e47e1@us.ibm.com>
Reviewed-by: Austin F Cui <d3f9654d68e47ce2b432124d933f90be1984ef68@ibm.com>
Reviewed-by: Zane C Shelley <fc96e127d170d6ac0b2af86a8a56f95974ff36e8@us.ibm.com>
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/140737
Tested-by: Jenkins OP Build CI <e610bd72261d3c0a48f1e8ae36832ab00774d426@us.ibm.com>
Tested-by: Jenkins Combined Simics CI <8312af3886dfc9bb930f549023677220805b137e@us.ibm.com>
Tested-by: Jenkins OP HW <0eb0fff262f74a65a832afbcc7b153e391c9a3fd@us.ibm.com>
Tested-by: FSP CI Jenkins <aa9e4d9ac7cd25905e9c0dd36d4150516e73dd86@us.ibm.com>
Tested-by: Hostboot CI <52521565bd8ea18a2b112942dce4f4220a97c2c8@us.ibm.com>

open-power/hostboot,open-power/hostboot,open-power/hostboot,open-power/hostboot,open-power/hostboot