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/Squash.cs

@@ -0,0 +1,54 @@
+using System.ComponentModel.DataAnnotations;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class Squash : Popup
+    {
+        public Models.Commit Head
+        {
+            get;
+            private set;
+        }
+
+        public Models.Commit Parent
+        {
+            get;
+            private set;
+        }
+
+        [Required(ErrorMessage = "Commit message is required!!!")]
+        public string Message
+        {
+            get => _message;
+            set => SetProperty(ref _message, value, true);
+        }
+
+        public Squash(Repository repo, Models.Commit head, Models.Commit parent)
+        {
+            _repo = repo;
+            _message = parent.FullMessage;
+            Head = head;
+            Parent = parent;
+            View = new Views.Squash() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Squashing ...";
+
+            return Task.Run(() =>
+            {
+                var succ = new Commands.Reset(_repo.FullPath, Parent.SHA, "--soft").Exec();
+                if (succ)
+                    succ = new Commands.Commit(_repo.FullPath, _message, true).Exec();
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return succ;
+            });
+        }
+
+        private readonly Repository _repo = null;
+        private string _message = string.Empty;
+    }
+}