Omit "Skipping <branch>" message when unneeded

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>

git_rebase_update.py

@@ -258,6 +258,8 @@ def main(args=None):
     git.freeze()  # just in case there are any local changes.
 
   skipped, branch_tree = git.get_branch_tree()
+  if opts.branches:
+    skipped = set(skipped).intersection(set(opts.branches))
   for branch in skipped:
     print 'Skipping %s: No upstream specified' % branch