project: reorganize the structure of the project.

* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.

src/ViewModels/AssumeUnchangedManager.cs

@@ -0,0 +1,38 @@
+using System.Threading.Tasks;
+
+using Avalonia.Collections;
+using Avalonia.Threading;
+
+namespace SourceGit.ViewModels
+{
+    public class AssumeUnchangedManager
+    {
+        public AvaloniaList<string> Files { get; private set; }
+
+        public AssumeUnchangedManager(string repo)
+        {
+            _repo = repo;
+            Files = new AvaloniaList<string>();
+
+            Task.Run(() =>
+            {
+                var collect = new Commands.AssumeUnchanged(_repo).View();
+                Dispatcher.UIThread.Invoke(() =>
+                {
+                    Files.AddRange(collect);
+                });
+            });
+        }
+
+        public void Remove(object param)
+        {
+            if (param is string file)
+            {
+                new Commands.AssumeUnchanged(_repo).Remove(file);
+                Files.Remove(file);
+            }
+        }
+
+        private readonly string _repo;
+    }
+}