Previously, `git reparent-branch` would call `git rebase-update
--no-fetch` effectively rebasing all branches. Limit the branches to
update to the one being moved and its dependants.

Change-Id: Ifb9f5b0ab4afe18f9c642ca47d683d9f3745bede
Reviewed-on: https://chromium-review.googlesource.com/c/1476393
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>