Circle CI step to fail if branch name is master

config.yml

      - run:
          name: Check for a branch named 'master'
          command: |
            git fetch --all --quiet --prune --prune-tags
            if [[ -n "$(git branch --all --list master */master)" ]]; then
              echo "A branch named 'master' was found. Please remove it."
              echo "$(git branch --all --list master */master)"
            fi
            [[ -z "$(git branch --all --list master */master)" ]]

Instructions:

Edit .circleci/config.yml and add the gist code to the steps: section under the checkout.

Examples of this in use

• Hyrax: (.circleci/config.yml), (PR Hyrax/#4904)
• Valkyrie: (.circleci/config.yml), (PR Valkyrie/#858)

Text for Issue:

NOTE: This applies if the repo uses Circle CI for integration testing.

Title

Add Circle CI step that fails if branch name is master

Description

### Descriptive summary

This repository’s default branch has already been renamed to `main` using GitHub’s renaming tool.  In order to preserve automatic redirection of links that reference the old branch name `master` to the new default `main` branch, a branch with the old name should not be recreated.

CircleCI can be used to prevent the recreation of the old default branch name by preventing PRs with a branch named `master` from being merged by causing a test failure during continuous integration.

### Rationale

Git's default "master" branch derives from "master/slave" jargon which perpetuates systemic racist language and systems (see email [Replacing "master" reference in git branch names](https://mail.gnome.org/archives/desktop-devel-list/2019-May/msg00066.html)). To uphold our Code of Conduct, we must move away from the term "master" in our technical language (as well as words like blacklist or whitelist).

### Expected behavior

If a PR is submitted with a branch named master, the continuous integration tests should fail.

### Actual behavior

If a PR is submitted with a branch named master, the continuous integration tests will not fail because of the branch name.

### Related work

Background on the renaming effort is available in the [working group notes](https://samvera.atlassian.net/wiki/x/b5IYHg).

Text for PR

Title

Adds Circle CI step that fails if branch name is master

Description

Fixes _____

This PR adds the Circle CI step that fails if the branch name is master.