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/Commit.cs

@@ -0,0 +1,21 @@
+using System.IO;
+
+namespace SourceGit.Commands
+{
+    public class Commit : Command
+    {
+        public Commit(string repo, string message, bool amend, bool allowEmpty = false)
+        {
+            var file = Path.GetTempFileName();
+            File.WriteAllText(file, message);
+
+            WorkingDirectory = repo;
+            Context = repo;
+            Args = $"commit --file=\"{file}\"";
+            if (amend)
+                Args += " --amend --no-edit";
+            if (allowEmpty)
+                Args += " --allow-empty";
+        }
+    }
+}