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/Merge.cs

@@ -0,0 +1,76 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class MergeMode
+    {
+        public string Name { get; set; }
+        public string Desc { get; set; }
+        public string Arg { get; set; }
+
+        public MergeMode(string n, string d, string a)
+        {
+            Name = n;
+            Desc = d;
+            Arg = a;
+        }
+    }
+
+    public class Merge : Popup
+    {
+        public string Source
+        {
+            get;
+            private set;
+        }
+
+        public string Into
+        {
+            get;
+            private set;
+        }
+
+        public List<MergeMode> Modes
+        {
+            get;
+            private set;
+        }
+
+        public MergeMode SelectedMode
+        {
+            get;
+            set;
+        }
+
+        public Merge(Repository repo, string source, string into)
+        {
+            _repo = repo;
+            Source = source;
+            Into = into;
+            Modes = new List<MergeMode>() {
+                new MergeMode("Default", "Fast-forward if possible", ""),
+                new MergeMode("No Fast-forward", "Always create a merge commit", "--no-ff"),
+                new MergeMode("Squash", "Use '--squash'", "--squash"),
+                new MergeMode("Don't commit", "Merge without commit", "--no-commit"),
+            };
+            SelectedMode = Modes[0];
+            View = new Views.Merge() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = $"Merging '{Source}' into '{Into}' ...";
+
+            return Task.Run(() =>
+            {
+                var succ = new Commands.Merge(_repo.FullPath, Source, SelectedMode.Arg, SetProgressDescription).Exec();
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return succ;
+            });
+        }
+
+        private readonly Repository _repo = null;
+    }
+}