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/QueryRepositoryRootPath.cs

@@ -0,0 +1,20 @@
+namespace SourceGit.Commands
+{
+    public class QueryRepositoryRootPath : Command
+    {
+        public QueryRepositoryRootPath(string path)
+        {
+            WorkingDirectory = path;
+            Args = "rev-parse --show-toplevel";
+            RaiseError = false;
+        }
+
+        public string Result()
+        {
+            var rs = ReadToEnd().StdOut;
+            if (string.IsNullOrEmpty(rs))
+                return null;
+            return rs.Trim();
+        }
+    }
+}