CheckoutFixture: update to expect undecorate tagname

Tag names are now recorded in the reflog without a leading `refs/tags/`,
beginning in libgit2/libgit2#4174

Author: ethomson

LibGit2Sharp.Tests/CheckoutFixture.cs

@@ -97,7 +97,7 @@ public void CanCheckoutAnExistingBranchByName(string branchName)
 
         [Theory]
         [InlineData("6dcf9bf", true, "6dcf9bf")]
-        [InlineData("refs/tags/lw", true, "refs/tags/lw")]
+        [InlineData("refs/tags/lw", true, "lw")]
         [InlineData("HEAD~2", true, "HEAD~2")]
         [InlineData("6dcf9bf", false, "6dcf9bf7541ee10456529833502442f385010c3d")]
         [InlineData("refs/tags/lw", false, "e90810b8df3e80c413d903f631643c716887138d")]
@@ -748,7 +748,7 @@ public void CheckingOutABranchDoesNotAlterBinaryFiles()
 
         [Theory]
         [InlineData("a447ba2ca8")]
-        [InlineData("refs/tags/lw")]
+        [InlineData("lw")]
         [InlineData("e90810^{}")]
         public void CheckoutFromDetachedHead(string commitPointer)
         {