Add challenge around "git push"

[alex-ball]

In episode three, we explain that our first, longer use of git push allows us to use a shorter form later, but we don't call back to that explanation when it happens. There are several ways we could do so, but I think using a challenge would be good as the lesson doesn't have very many of them.

Closes #166.

[github-actions]

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⏱️ Updated at 2025-08-14 08:21:03 +0000

[srerickson]

I'm offering slightly more in-depth language for the solution:

Branches can be configured with an "upstream" branch (a branch on a remote
repository), which is used automatically for `push` and `pull` operations. The
`main` branch's upstream wasn't set when we created it. The `-u` (or
`--set-upstream`) flag tells `git push` which remote branch to use; it also sets
the remote branch as the local branch's upstream. Once we've set it, we don't
need to specify the remote branch in future `push` and `pull` operations.

Feel free to incorporate it, or not. Otherwise this, looks good!

[alex-ball]

Thank you for the more thorough explanation, which I've incorporated.

I think this reinforces the point that we need to find somewhere to explain what a branch is, as touched on in #54.