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/QueryFileContent.cs

@@ -0,0 +1,28 @@
+using System.Text;
+
+namespace SourceGit.Commands
+{
+    public class QueryFileContent : Command
+    {
+        public QueryFileContent(string repo, string revision, string file)
+        {
+            WorkingDirectory = repo;
+            Context = repo;
+            Args = $"show {revision}:\"{file}\"";
+        }
+
+        public string Result()
+        {
+            Exec();
+            return _builder.ToString();
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _builder.Append(line);
+            _builder.Append('\n');
+        }
+
+        private readonly StringBuilder _builder = new StringBuilder();
+    }
+}