Fix resolving HEAD reference if it's a packed ref

[filipnavara]

The issue is that we have a repository that has HEAD pointing containing ref: refs/heads/netcore/master. All references are packed so to resolve refs/heads/netcore/master it's necessary to looking into the packed refs and not just on the file system.

Fixes #602

[filipnavara]

Eh, looks like the fix will need a bit of tweaking. At least we know the underlying cause though :)

[azure-pipelines]

Commenter does not have sufficient privileges for PR 613 in repo dotnet/Nerdbank.GitVersioning

[AArnott]

Thanks for contributing, and especially for the repro steps. Are you planning to add the test case or should I find some time to do that?

[AArnott]

Also, we need to rebase your PR to the v3.4 branch since we need to fix that version, and master targets v3.5. Would you like to do that or shall I and force-push to your source branch?

[filipnavara]

You will likely have better idea how to structure the test case than me so I would prefer if you look into it. I've uploaded an example repository in the linked issue so it should be fairly trivial to incorporate it somehow.

Feel free to push to the branch as much as necessary. I can rebase it on top of the 3.4 branch but at the moment I am not anywhere near a computer 😅

It also got me thinking that aside from the actual problem that I fixed it would have been reasonable to update GetBuildVersion to either fail or at least issue a warning in the MSBuild task. Now it silently produces a version without the GIT fields in ThisAssembly and with the base version instead of the calculated one.

[filipnavara]

Thanks!

[AArnott]

Thank you very much for contributing the fix!