`clean-conversation-headers` should also consider `master` as a default #3987

<img width="829" alt="Screen Shot" src="https://user-images.githubusercontent.com/1402241/108141825-167caf80-708a-11eb-86e9-a96d796e638a.png">

master here refers to the default branch. Since this is a common occurrence, we should just assume it was the old default branch

Like https://github.com/sindresorhus/refined-github/issues/3910 and maybe https://github.com/sindresorhus/refined-github/commit/aa6659aad95973fb9e32dd2b7cb4c4273dd100f6

Demo URL: https://github.com/parcel-bundler/parcel/pull/78