Enable correct PR versioning for Buildkite (#3013)

* Enable correct PR versioning for Buildkite

* Fix formatting issues

Author: santhonisz

src/GitVersion.Core.Tests/BuildAgents/BuildKiteTests.cs

@@ -52,6 +52,7 @@ public void GetCurrentBranchShouldHandleBranches()
     {
         // Arrange
         this.environment.SetEnvironmentVariable("BUILDKITE_BRANCH", MainBranch);
+        this.environment.SetEnvironmentVariable("BUILDKITE_PULL_REQUEST", "false");
 
         // Act
         var result = this.buildServer.GetCurrentBranch(false);
@@ -60,6 +61,20 @@ public void GetCurrentBranchShouldHandleBranches()
         result.ShouldBe(MainBranch);
     }
 
+    [Test]
+    public void GetCurrentBranchShouldHandlePullRequests()
+    {
+        // Arrange
+        this.environment.SetEnvironmentVariable("BUILDKITE_BRANCH", "feature/new");
+        this.environment.SetEnvironmentVariable("BUILDKITE_PULL_REQUEST", "55");
+
+        // Act
+        var result = this.buildServer.GetCurrentBranch(false);
+
+        // Assert
+        result.ShouldBe("refs/pull/55/head");
+    }
+
     [Test]
     public void GetSetParameterMessageShouldReturnEmptyArray()
     {

src/GitVersion.Core/BuildAgents/BuildKite.cs

@@ -21,7 +21,20 @@ public override string GenerateSetVersionMessage(VersionVariables variables) =>
     public override string[] GenerateSetParameterMessage(string name, string value) =>
         Array.Empty<string>(); // There is no equivalent function in BuildKite.
 
-    public override string? GetCurrentBranch(bool usingDynamicRepos) => Environment.GetEnvironmentVariable("BUILDKITE_BRANCH");
+    public override string? GetCurrentBranch(bool usingDynamicRepos)
+    {
+        var pullRequest = Environment.GetEnvironmentVariable("BUILDKITE_PULL_REQUEST");
+        if (string.IsNullOrEmpty(pullRequest) || pullRequest == "false")
+        {
+            return Environment.GetEnvironmentVariable("BUILDKITE_BRANCH");
+        }
+        else
+        {
+            // For pull requests BUILDKITE_BRANCH refers to the head, so adjust the
+            // branch name for pull request versioning to function as expected
+            return string.Format("refs/pull/{0}/head", pullRequest);
+        }
+    }
 
     public override bool PreventFetch() => true;
 }