Add a test that reproduces the reported issue to #17399

[tkelman]

@wildart I got impatient, please review. The first few times I tried, LibGit2.add! did not seem to work, but now it's apparently okay.

The first 2 commits here are the same as #17399, just rebased. The third commit adds a test that reproduces the actual issue, meaning it fails if you run the new test without your changes to base here.

[wildart]

Commit is done into the checked out repo Example3, but it should be to the remote branch of Example.

But mine is better 😉, I just pushed #17399.

[tkelman]

Commit is done into the checked out repo Example3, but it should be to the remote branch of Example.

No, the issue is a WARNING: Cannot perform fast-forward merge. because the local commits on the pre-checkout package conflict with the checked-out branch.

[tkelman]

Let's just do both.

[wildart]

If it can't fast-forward merge, it's a trouble. The branch is going to be created but never positioned correctly.

[tkelman]

It really shouldn't be doing a merge at all, we asked it to do a checkout.

[wildart]

Problem is that git checkout is not just checkout particular branch or commit. It's much more. And, when you checkout a remote branch, you want to FF merge changes on that branch.

[tkelman]

I'm going to merge this one, since it tests more scenarios.