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/Version.cs

@@ -0,0 +1,19 @@
+namespace SourceGit.Commands
+{
+    public class Version : Command
+    {
+        public Version()
+        {
+            Args = "--version";
+            RaiseError = false;
+        }
+
+        public string Query()
+        {
+            var rs = ReadToEnd();
+            if (!rs.IsSuccess || string.IsNullOrWhiteSpace(rs.StdOut))
+                return string.Empty;
+            return rs.StdOut.Trim().Substring("git version ".Length);
+        }
+    }
+}