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/Cleanup.cs

@@ -0,0 +1,36 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class Cleanup : Popup
+    {
+        public Cleanup(Repository repo)
+        {
+            _repo = repo;
+            View = new Views.Cleanup() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Cleanup (GC & prune) ...";
+
+            return Task.Run(() =>
+            {
+                new Commands.GC(_repo.FullPath, SetProgressDescription).Exec();
+
+                var lfs = new Commands.LFS(_repo.FullPath);
+                if (lfs.IsEnabled())
+                {
+                    SetProgressDescription("Run LFS prune ...");
+                    lfs.Prune(SetProgressDescription);
+                }
+
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return true;
+            });
+        }
+
+        private readonly Repository _repo = null;
+    }
+}