Sourcebot doesn't index the "default" branch if it has been changed

[jerrykan]

We have noticed when using Sourcebot with our self-hosted GitLab instance that if the default branch of a repository changes Sourcebot doesn't start indexing the new default branch. Instead Sourcebot continues to index what was the default branch when the repository was first indexed.

What I would expect to happen if the default branch is changed (via <repo> -> Settings -> Repository -> Branch defaults -> default Branch) is that Sourcebot would index the new default branch the next time the repository is indexed.

I am aware of issue #29, but I believe this is a separate problem.