fix invalid already rendered folder error

fixes #246
closes #391

commit 2c37b20d129a231a16d2e64cbf2f007aa73e40c4
Author: enricosada <enrico@sada.io>
Date:   Wed Apr 29 18:57:04 2015 +0200

    fix invalid already rendered folder error

    fix invalid already rendered folder error when a project contains
    folders with same name

commit b8d03011db57b3b5df66c31d3a1564352c947cd1
Author: enricosada <enrico@sada.io>
Date:   Wed May 6 23:30:12 2015 +0200

    add regression tests

Author: enricosada

vsintegration/src/unittests/Tests.ProjectSystem.RoundTrip.fs

@@ -100,6 +100,17 @@ type RoundTrip() =
                              CompileItem @"A\qux.fs"]
         this.``FsprojRoundtrip.PositiveTest``(MSBuildItems origItems, MSBuildItems expectedItems)
 
+    [<Test>]
+    member public this.``FsprojRoundTrip.Regression.FoldersWithSameName``() =
+        let items = MSBuildItems [CompileItem @"First\Second\bar.fs"
+                                  CompileItem @"Second\qux.fs"]
+        this.``FsprojRoundtrip.PositiveTest``(items, items)
+
+    [<Test>]
+    member public this.``FsprojRoundTrip.Regression.FoldersWithSameName2``() =
+        let items = MSBuildItems [CompileItem @"First\First\bar.fs"]
+        this.``FsprojRoundtrip.PositiveTest``(items, items)
+
     member this.``Fsproj.NegativeTest``(items : MSBuildItems) =
         DoWithTempFile "Test.fsproj" (fun file ->
             File.AppendAllText(file, TheTests.SimpleFsprojText([], [], items.ToString()))

vsintegration/src/vs/FsPkgs/FSharp.Project/FS/MSBuildUtilities.fs

@@ -120,7 +120,7 @@ type internal MSBuildUtilities() =
                 // push folder
                 Inc(curPathParts)
                 curPathParts.Add(pathParts.[curPathParts.Count]) // e.g. transition from A\ to A\D\E\bar.fs
-                if not(alreadyRenderedFolders.Add(curPathParts)) && throwIfCannotRender then
+                if not(alreadyRenderedFolders.Add(new List<string>(curPathParts))) && throwIfCannotRender then
                     raise <| new InvalidOperationException(String.Format(FSharpSR.GetString(FSharpSR.ProjectRenderFolderMultiple), projectNode.ProjectFile, bi.Include))
             Inc(curPathParts)
             if bi.ItemType = ProjectFileConstants.Folder then