topic: complete "//" format support in experimental.topic.linear-merge code This code is used for merging topics into bare branches (with "allow-from-bare-branch"), and it modifies branch name on a specific revision to be different from branch name on the merge destination. This allows merging the two changesets even when they belong to the same branch. Let's use formatfqbn() for consistency. The format doesn't matter much as long as the result is different from bare branch name. Topic namespace is ignored here by design, there needs to be a topic set for this linear-merge mode to work.