feature<Fast-Forward>: allow fast-forward on local branch that not checked out.

src/Commands/Fetch.cs

@@ -29,6 +29,22 @@ namespace SourceGit.Commands {
             AutoFetch.MarkFetched(repo);
         }
 
+        public Fetch(string repo, string remote, string localBranch, string remoteBranch, Action<string> outputHandler) {
+            Cwd = repo;
+            TraitErrorAsOutput = true;
+
+            var sshKey = new Config(repo).Get($"remote.{remote}.sshkey");
+            if (!string.IsNullOrEmpty(sshKey)) {
+                Envs.Add("GIT_SSH_COMMAND", $"ssh -i '{sshKey}'");
+                Args = "";
+            } else {
+                Args = "-c credential.helper=manager-core ";
+            }
+
+            Args += $"fetch --progress --verbose {remote} {remoteBranch}:{localBranch}";
+            handler = outputHandler;
+        }
+
         public override void OnReadline(string line) {
             handler?.Invoke(line);
         }

src/Views/Popups/FastForwardWithoutCheckout.xaml

@@ -0,0 +1,10 @@
+<controls:PopupWidget 
+    x:Class="SourceGit.Views.Popups.FastForwardWithoutCheckout"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
+    xmlns:controls="clr-namespace:SourceGit.Views.Controls"
+    mc:Ignorable="d" 
+    d:DesignWidth="500" Height="100">
+</controls:PopupWidget>

src/Views/Popups/FastForwardWithoutCheckout.xaml.cs

@@ -0,0 +1,46 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.Views.Popups {
+    /// <summary>
+    ///     对于不是当前分支的本地分支，Fast-Forward
+    /// </summary>
+    public partial class FastForwardWithoutCheckout : Controls.PopupWidget {
+        private string repo = null;
+        private string remote = null;
+        private string localBranch = null;
+        private string remoteBranch = null;
+        private bool isValid = false;
+
+        public FastForwardWithoutCheckout(string repo, string branch, string upstream) {
+            int idx = upstream.IndexOf('/');
+            if (idx < 0 || idx == upstream.Length - 1) {
+                Models.Exception.Raise($"Invalid upstream: {upstream}");
+                return;
+            }
+
+            this.repo = repo;
+            this.remote = upstream.Substring(0, idx);
+            this.localBranch = branch;
+            this.remoteBranch = upstream.Substring(idx+1);
+            this.isValid = true;
+
+            InitializeComponent();
+        }
+
+        public override string GetTitle() {
+            return App.Text("Fetch.Title");
+        }
+
+        public override Task<bool> Start() {
+            return Task.Run(() => {
+                if (isValid) {
+                    Models.Watcher.SetEnabled(repo, false);
+                    new Commands.Fetch(repo, remote, localBranch, remoteBranch, UpdateProgress).Exec();
+                    Models.Watcher.SetEnabled(repo, true);
+                }
+                
+                return true;
+            });
+        }
+    }
+}

src/Views/Widgets/Dashboard.xaml.cs

@@ -654,7 +654,7 @@ namespace SourceGit.Views.Widgets {
                     var upstream = branch.Upstream.Substring(13);
                     var fastForward = new MenuItem();
                     fastForward.Header = App.Text("BranchCM.FastForward", upstream);
-                    fastForward.IsEnabled = !string.IsNullOrEmpty(branch.UpstreamTrackStatus);
+                    fastForward.IsEnabled = !string.IsNullOrEmpty(branch.UpstreamTrackStatus) && branch.UpstreamTrackStatus.IndexOf('↑') < 0;
                     fastForward.Click += (o, e) => {
                         new Popups.Merge(repo.Path, upstream, branch.Name).ShowAndStart();
                         e.Handled = true;
@@ -683,6 +683,21 @@ namespace SourceGit.Views.Widgets {
                     e.Handled = true;
                 };
                 menu.Items.Add(checkout);
+
+                if (!string.IsNullOrEmpty(branch.Upstream)) {
+                    var upstream = branch.Upstream.Substring(13);
+                    var fastForward = new MenuItem();
+                    fastForward.Header = App.Text("BranchCM.FastForward", upstream);
+                    fastForward.IsEnabled = !string.IsNullOrEmpty(branch.UpstreamTrackStatus) && branch.UpstreamTrackStatus.IndexOf('↑') < 0;
+                    fastForward.Click += (o, e) => {
+                        new Popups.FastForwardWithoutCheckout(repo.Path, branch.Name, upstream).ShowAndStart();
+                        e.Handled = true;
+                    };
+
+                    menu.Items.Add(new Separator());
+                    menu.Items.Add(fastForward);
+                }
+
                 menu.Items.Add(new Separator());
                 menu.Items.Add(push);