feature: allow fetch the latest remote changes into local branch which is not current branch (#617)

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/FetchInto.cs

@@ -0,0 +1,42 @@
+using System.Threading.Tasks;
+
+namespace SourceGit.ViewModels
+{
+    public class FetchInto : Popup
+    {
+        public Models.Branch Local
+        {
+            get;
+            private set;
+        }
+
+        public Models.Branch Upstream
+        {
+            get;
+            private set;
+        }
+
+        public FetchInto(Repository repo, Models.Branch local, Models.Branch upstream)
+        {
+            _repo = repo;
+            Local = local;
+            Upstream = upstream;
+            View = new Views.FetchInto() { DataContext = this };
+        }
+
+        public override Task<bool> Sure()
+        {
+            _repo.SetWatcherEnabled(false);
+            ProgressDescription = "Fast-Forward ...";
+
+            return Task.Run(() =>
+            {
+                new Commands.Fetch(_repo.FullPath, Local, Upstream, SetProgressDescription).Exec();
+                CallUIThread(() => _repo.SetWatcherEnabled(true));
+                return true;
+            });
+        }
+
+        private readonly Repository _repo = null;
+    }
+}