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/Commands/Checkout.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace SourceGit.Commands
+{
+    public class Checkout : Command
+    {
+        public Checkout(string repo)
+        {
+            WorkingDirectory = repo;
+            Context = repo;
+        }
+
+        public bool Branch(string branch, Action<string> onProgress)
+        {
+            Args = $"checkout --progress {branch}";
+            TraitErrorAsOutput = true;
+            _outputHandler = onProgress;
+            return Exec();
+        }
+
+        public bool Branch(string branch, string basedOn, Action<string> onProgress)
+        {
+            Args = $"checkout --progress -b {branch} {basedOn}";
+            TraitErrorAsOutput = true;
+            _outputHandler = onProgress;
+            return Exec();
+        }
+
+        public bool File(string file, bool useTheirs)
+        {
+            if (useTheirs)
+            {
+                Args = $"checkout --theirs -- \"{file}\"";
+            }
+            else
+            {
+                Args = $"checkout --ours -- \"{file}\"";
+            }
+
+            return Exec();
+        }
+
+        public bool FileWithRevision(string file, string revision)
+        {
+            Args = $"checkout {revision} -- \"{file}\"";
+            return Exec();
+        }
+
+        public bool Files(List<string> files)
+        {
+            StringBuilder builder = new StringBuilder();
+            builder.Append("checkout -f -q --");
+            foreach (var f in files)
+            {
+                builder.Append(" \"");
+                builder.Append(f);
+                builder.Append("\"");
+            }
+            Args = builder.ToString();
+            return Exec();
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _outputHandler?.Invoke(line);
+        }
+
+        private Action<string> _outputHandler;
+    }
+}