If the change URL isn't available (e.g. because we don't have gerrit.canonicalWebUrl configured) we cannot pass down the --change-url flag to certain hooks. Safeguard the code from that failure case. Bug: issue 641 Change-Id: Ib26ec8759e0090aadfc815c17fe7db0f299c640e Signed-off-by: Shawn O. Pearce <sop@google.com>