Using --no-squash resulted in an exception being thrown, because issue
number is not set in that case.

Change-Id: Iaf0d7eb05851eba0d5231640c077b845bca486ee
Reviewed-on: https://chromium-review.googlesource.com/505509Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>