GitVersion: [Bug] Tag checkout on GitHub Actions generates unexpected branch

We have a GitHub Action that triggers on tag push. When it runs, GitVersion generates a version like this: 1.3.0-tags-1-2-1.1+6. The 1.2.1 tag is defined on the tip of the release-1.2 branch.

When I run GitVersion locally, it generates the expected version 1.2.1. When I configure my local environment to replicate the GitHub Actions environment, I do get the same incorrect version number.

Expected Behavior

When checking out a tag in GitHub actions that relates to a version number, that version number should be generated.

> git branch
* (HEAD detached at 1.2.1)

> dotnet-gitversion
{
  // ...
  "SemVer": "1.2.1"
  // ...
}

Actual Behavior

Normalization and checking out the GITHUB_REF as a branch is causing an unexpected branch to be created and checked out and that is giving us an incorrect version number.

> git branch
* (HEAD detached at 1.2.1)

> SET GITHUB_ACTIONS=true

> SET GITHUB_REF=refs/tags/1.2.1

> dotnet-gitversion
{
  // ...
  "SemVer": "1.3.0-tags-1-2-1.1"
  // ...
}

> git branch
  master
  release-1.0
  release-1.2
  release-1.3
* tags/1.2.1

Possible Fix

The GITHUB_REF environment variable is set to the tag ref (i.e. refs/tags/1.2.1) but this gets treated like a branch name. Ultimately this ref gets mangled into a canonical name like this refs/heads/tags/1.2.1, and turned into a branch named tags/1.2.1. This branch name is used in the SemVer version.

Steps to Reproduce

NOTE: The repo experiencing this issue is this one.

This is the cmd script that I have used to reproduce this locally

@REM Pretending to be GitHub Actions
set GITHUB_ACTIONS=true
set GITHUB_REF=refs/tags/1.2.1

@REM Simulate actions/checkout@v2.3.4 with fetch-depth 0
git init .
git remote add origin https://github.com/Particular/NServiceBus.AzureFunctions.InProcess.ServiceBus
git -c protocol.version=2 fetch --prune --progress --no-recurse-submodules origin +refs/heads/*:refs/remotes/origin/* +refs/tags/*:refs/tags/*
git checkout --progress --force refs/tags/1.2.1

@REM Build
dotnet build src --configuration Release

Context

Your Environment

• Version Used: 5.6.9 (also tested with 5.7.0+Branch.main.Sha.8d177c6d666c8eeb1c6a6a2c71fd4b78741137d21 as installed global tool)
• Operating System and version (Windows 10, Ubuntu 18.04): GitHub Actions with windows-2019
• Link to your project: https://github.com/Particular/NServiceBus.AzureFunctions.InProcess.ServiceBus
• Link to your CI build (if appropriate): https://github.com/Particular/NServiceBus.AzureFunctions.InProcess.ServiceBus/blob/release-1.2/.github/workflows/release.yml