Workaround for Unity VersionControl.Provider.Checkout() regression

According to the user, calling VersionControl.Provider.Checkout(
string, CheckoutMode) may throw exception in certain version of
Unity due to Unity regression, while VersionControl.Provider.Checkout(
AssetList, CheckoutMode) works properly.

Fixed #359
Bug: 154264996
Change-Id: I011595274dbb83d202f6b0bf461de530f6f802ed

Change-Id: I5970acab818195d3ce721c4a7978fb935cae43cd

Author: chkuang-g

source/VersionHandlerImpl/src/FileUtils.cs

@@ -326,14 +326,19 @@ public static bool CheckoutFile(string path, Logger logger) {
                     (!UnityEditor.VersionControl.Provider.requiresNetwork ||
                      UnityEditor.VersionControl.Provider.onlineState ==
                      UnityEditor.VersionControl.OnlineState.Online)) {
+
+                    // Some versions of Unity seem to have bug to convert "string path" to
+                    // "AssetList assets" (See #359). Generate it in advance as a workaround.
+                    var assetList = new UnityEditor.VersionControl.AssetList();
+                    assetList.Add(new UnityEditor.VersionControl.Asset(path));
                     // Unity 2019.1+ broke backwards compatibility of Checkout() by adding an
                     // optional argument to the method so we dynamically invoke the method to add
                     // the optional
                     // argument for the Unity 2019.1+ overload at runtime.
                     var task = (UnityEditor.VersionControl.Task)VersionHandler.InvokeStaticMethod(
                         typeof(UnityEditor.VersionControl.Provider),
                         "Checkout",
-                        new object[] { path, UnityEditor.VersionControl.CheckoutMode.Exact },
+                        new object[] { assetList, UnityEditor.VersionControl.CheckoutMode.Exact },
                         namedArgs: null);
                     task.Wait();
                     if (!task.success) {