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/Add.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using System.Text;
+
+namespace SourceGit.Commands
+{
+    public class Add : Command
+    {
+        public Add(string repo, List<Models.Change> changes = null)
+        {
+            WorkingDirectory = repo;
+            Context = repo;
+
+            if (changes == null || changes.Count == 0)
+            {
+                Args = "add .";
+            }
+            else
+            {
+                var builder = new StringBuilder();
+                builder.Append("add --");
+                foreach (var c in changes)
+                {
+                    builder.Append(" \"");
+                    builder.Append(c.Path);
+                    builder.Append("\"");
+                }
+                Args = builder.ToString();
+            }
+        }
+    }
+}