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/Pull.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace SourceGit.Commands
+{
+    public class Pull : Command
+    {
+        public Pull(string repo, string remote, string branch, bool useRebase, Action<string> outputHandler)
+        {
+            _outputHandler = outputHandler;
+            WorkingDirectory = repo;
+            Context = repo;
+            TraitErrorAsOutput = true;
+
+            var sshKey = new Config(repo).Get($"remote.{remote}.sshkey");
+            if (!string.IsNullOrEmpty(sshKey))
+            {
+                Args = $"-c core.sshCommand=\"ssh -i '{sshKey}'\" ";
+            }
+            else
+            {
+                Args = "-c credential.helper=manager ";
+            }
+
+            Args += "pull --verbose --progress --tags ";
+            if (useRebase)
+                Args += "--rebase ";
+            Args += $"{remote} {branch}";
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _outputHandler?.Invoke(line);
+        }
+
+        private readonly Action<string> _outputHandler;
+    }
+}