tools/push-to-trunk.sh: Add a check whether grepping for the just-committed SVN revision number succeeded.

R=yangguo@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10828210

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12279 ce2b1a6d-e550-0410-aec6-3dcde31c8c00