Issue 3518: "Submitted Together" does not work on nonexistent branch
Project Member Reported by dborowitz@google.com, Aug 12, 2015
Steps to reproduce:
1. Create a new project.
2. Push 2 dependent changes to review.
3. Open the second (tipmost) change.

Expected results:
2 changes in "Submitted Together" tab

Observed results:
No "Submitted Together" tab

The problem is completeChangeSetWithoutTopic skips the RevWalk if the dest branch is missing. It should do the walk, just skip marking the uninteresting commit.