Do foundational work to support to support gitlab-org/gitlab default branch

Overview

gitlab-com/gl-infra/delivery#1357 (closed) plans to rename gitlab-org/gitlab default branch from master to main, the timeline is still in the works (it might happen during the first quarter of 2021) but when it happens we should be prepared.

Let's use this issue to discuss what needs to be done on GitLab Shell before the switch.

Proposal

I'm not certain this project depends on a hardcoded master branch for gitlab-org/gitlab. From a quick search, seems it only references master on a doc file:

./README.md:75:[`GITLAB_SHELL_VERSION`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/GITLAB_SHELL_VERSION).

Rough plan could be as follow:

  1. Identify what areas of the GitLab Shell codebase depend on a hardcoded master branch for gitlab-org/gitlab
  2. Adjust the corresponding files (if any)