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/Fetch.cs

@@ -24,16 +24,6 @@ namespace SourceGit.Commands
             Args += remote;
         }
 
-        public Fetch(string repo, string remote, string localBranch, string remoteBranch, Action<string> outputHandler)
-        {
-            _outputHandler = outputHandler;
-            WorkingDirectory = repo;
-            Context = repo;
-            TraitErrorAsOutput = true;
-            SSHKey = new Config(repo).Get($"remote.{remote}.sshkey");
-            Args = $"fetch --progress --verbose {remote} {remoteBranch}:{localBranch}";
-        }
-
         protected override void OnReadline(string line)
         {
             _outputHandler?.Invoke(line);

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;
+    }
+}

src/ViewModels/FastForwardWithoutCheckout.cs

@@ -31,7 +31,7 @@ namespace SourceGit.ViewModels
 
             return Task.Run(() =>
             {
-                new Commands.Fetch(_repo.FullPath, To.Remote, Local.Name, To.Name, SetProgressDescription).Exec();
+                new Commands.UpdateRef(_repo.FullPath, Local.FullName, To.FullName, SetProgressDescription).Exec();
                 CallUIThread(() => _repo.SetWatcherEnabled(true));
                 return true;
             });