Ensure merges to different branches can't roll up together

[tgross35]

Apparently bors allows this "feature", though it probably shouldn't :) rust-lang/rust#139765

[ChrisDenton]

Oh oops. Tbf, it didn't actually get into the rollup due to not being mergeable on the master branch.

[tgross35]

I guess it would have tried merging all of beta into master? Safety mechanism ha. Just figured it was worth making sure new bors has a testcase for this.