Add Circle CI step that fails if branch name is master

[cjcolvar]

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). 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.