When calling `git rebase-update <branches>`, don't emit messages about
skipping branches which are not in the list of branches to be rebased.

R=iannucci@chromium.org

Change-Id: Ia5135d2e794646aaec68bca1aa64f4a73c98c186
Reviewed-on: https://chromium-review.googlesource.com/c/1476394
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>