Product discovery: Global code search including more than just master branch

Global code search only works on the master branch. It should be possible to specify additional branches to be added to the code search facility.

GitLab flow generally implies working with a master branch and temporary branches that go away after their content is merged into the master. But not all of our customers work this way. We have had interest from prospects in indexing of additional branches. This would be a good differentiator vs. some of our competitors, none of whom offer this. Yes - it requires additional work for the server/indexing processes. But if we allow configuration by specifying branches to index we can provide what customers want without indexing everything on every branch.