Fix test on newer versions of libgit2sharp

See libgit2/libgit2sharp#1412

Author: AArnott

src/NerdBank.GitVersioning.Tests/GitExtensionsTests.cs

@@ -95,7 +95,7 @@ public void GetVersionHeight_VersionJsonHasUnrelatedHistory()
 
         // Also emulate case of where the related project.json was just changed to conform,
         // but not yet checked in.
-        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());
     }