Non-default branches are copies instead of diffs

In my class, I create assignments from repository templates that have a `main` branch and a `solution` branch. The `solution` branch is one commit ahead of the `main` branch so that students can click on `solution`'s commit to see a diff of the solution. At least, that is how it worked last year.

This year, GitHub Classroom changed such that there's a new option "Copy the default branch only". When this option is unchecked, the `solution` branch is _copied_ as if it were an entirely new branch (parallel to `main`). As a result, the diff is lost! This makes it very difficult for students to see the parts of the code that are added/removed/changed in the solution.

(P.S. Is there another (better) approach to providing solutions to assignments?)

Reproduce the assignment creation flow with a template containing main and a one-commit-ahead solution branch, comparing behavior when "Copy the default branch only" is unchecked. Trace the branch-copying entry point and verify that the resulting solution branch retains ancestry so students can view the intended diff.

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