Do a FF-pull when updating PR from same repo.

Fixes #578.

Author: Steven Kirk

src/GitHub.App/Services/PullRequestService.cs

@@ -90,6 +90,16 @@ public IObservable<bool> IsCleanForCheckout(ILocalRepositoryModel repository)
             return Observable.Return(!repo.RetrieveStatus().IsDirty);
         }
 
+        public IObservable<Unit> Pull(ILocalRepositoryModel repository)
+        {
+            return Observable.Defer(() =>
+            {
+                var repo = gitService.GetRepository(repository.LocalPath);
+                var refspec = string.Format(CultureInfo.InvariantCulture, "{0}:{0}", repo.Head.FriendlyName);
+                return gitClient.Fetch(repo, "origin", new[] { refspec }).ToObservable();
+            });
+        }
+
         public IObservable<Unit> FetchAndCheckout(ILocalRepositoryModel repository, int pullRequestNumber, string localBranchName)
         {
             return DoFetchAndCheckout(repository, pullRequestNumber, localBranchName).ToObservable();

src/GitHub.App/ViewModels/PullRequestDetailViewModel.cs

@@ -485,7 +485,7 @@ IObservable<Unit> DoCheckout(object unused)
             switch (CheckoutMode)
             {
                 case CheckoutMode.NeedsPull:
-                    operation = pullRequestsService.FetchAndCheckout(repository, Number, repository.CurrentBranch.Name);
+                    operation = pullRequestsService.Pull(repository);
                     break;
                 case CheckoutMode.Fetch:
                     operation = pullRequestsService

src/GitHub.Exports.Reactive/Services/IPullRequestService.cs

@@ -29,6 +29,12 @@ IObservable<IPullRequestModel> CreatePullRequest(IRepositoryHost host,
         /// <returns></returns>
         IObservable<Unit> FetchAndCheckout(ILocalRepositoryModel repository, int pullRequestNumber, string localBranchName);
 
+        /// <summary>
+        /// Carries out a pull on the current branch.
+        /// </summary>
+        /// <param name="repository">The repository.</param>
+        IObservable<Unit> Pull(ILocalRepositoryModel repository);
+
         /// <summary>
         /// Calculates the name of a local branch for a pull request avoiding clashes with existing branches.
         /// </summary>