Dynamically handle `master`, `main` as default branch

[akphi]

Gitlab starts using main as their default branch. Right now we hard-coded this to master

The strategy we need to adopt is:

1. Long Term Dynamically handle this with a new logic centralized at the getBranch() function: i.e. Gitlab project API provides information about which is the default branch - See https://docs.gitlab.com/ee/api/projects.html#get-single-project
2. Short Term Tell user to configure the default branch on Gitlab at group level to master