Fix another test that doesn't sort commits properly

Author: AArnott

src/NerdBank.GitVersioning.Tests/GitExtensionsTests.cs

@@ -118,7 +118,7 @@ public void GetVersionHeight_VersionJsonHasParsingErrorsInHistory()
         Assert.Equal(1, this.Repo.GetVersionHeight());
 
         // And emulate fixing it without having checked in yet.
-        this.Repo.Reset(ResetMode.Mixed, this.Repo.Head.Commits.Skip(1).First());
+        this.Repo.Reset(ResetMode.Mixed, this.Repo.Head.Tip.Parents.Single());
         Assert.Equal(0, this.Repo.GetVersionHeight());
     }