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

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;
+            });
+        }
+    }
+}