refactor: use git update-ref $LOCAL_BRANCH $REMOTE_BRANCH instead of git fetch $REMOTE $LOCAL_BRANCH $REMOTE_BRANCH to fast-forward local branch without checkout it first.

src/Commands/UpdateRef.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace SourceGit.Commands
+{
+    public class UpdateRef : Command
+    {
+        public UpdateRef(string repo, string refName, string toRevision, Action<string> outputHandler)
+        {
+            _outputHandler = outputHandler;
+
+            WorkingDirectory = repo;
+            Context = repo;
+            Args = $"update-ref {refName} {toRevision}";
+        }
+
+        protected override void OnReadline(string line)
+        {
+            _outputHandler?.Invoke(line);
+        }
+
+        private Action<string> _outputHandler;
+    }
+}