git-svn rebase will always output 'Current branch <> is up-to-date' when the
branch is up-to-date, even if you pass the --quiet flag(s). This change
specifically squelches that message.

Review URL: http://codereview.chromium.org/555183

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@37696 0039d316-1c4b-4281-b951-d872f2087c98