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/Statistics.cs

@@ -0,0 +1,39 @@
+using System;
+
+namespace SourceGit.Commands
+{
+    public class Statistics : Command
+    {
+        public Statistics(string repo)
+        {
+            _statistics = new Models.Statistics();
+
+            WorkingDirectory = repo;
+            Context = repo;
+            Args = $"log --date-order --branches --remotes --since=\"{_statistics.Since()}\" --pretty=format:\"%ct$%cn\"";
+        }
+
+        public Models.Statistics Result()
+        {
+            Exec();
+            _statistics.Complete();
+            return _statistics;
+        }
+
+        protected override void OnReadline(string line)
+        {
+            var dateEndIdx = line.IndexOf('$', StringComparison.Ordinal);
+            if (dateEndIdx == -1)
+                return;
+
+            var dateStr = line.Substring(0, dateEndIdx);
+            var date = 0.0;
+            if (!double.TryParse(dateStr, out date))
+                return;
+
+            _statistics.AddCommit(line.Substring(dateEndIdx + 1), date);
+        }
+
+        private readonly Models.Statistics _statistics = null;
+    }
+}