Add support for refreshing an existing collection

To support refreshing data from the server without having to clear the
whole collection and redo inserting and sorting and filtering all over
again, we can just pass an existing collection to `ModelService`, so it
will set up the new query and update the existing collection.

Author: shana

src/GitHub.App/Services/ModelService.cs

@@ -135,7 +135,8 @@ public IObservable<AccountCacheItem> GetUserFromCache()
             return Observable.Defer(() => hostCache.GetObject<AccountCacheItem>("user"));
         }
 
-        public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryModel repo)
+        public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryModel repo,
+            [AllowNull]ITrackingCollection<IPullRequestModel> collection = null)
         {
             // Since the api to list pull requests returns all the data for each pr, cache each pr in its own entry
             // and also cache an index that contains all the keys for each pr. This way we can fetch prs in bulk
@@ -146,7 +147,8 @@ public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryM
             var keyobs = GetUserFromCache()
                 .Select(user => string.Format(CultureInfo.InvariantCulture, "{0}|{1}|pr", user.Login, repo.Name));
 
-            var col = new TrackingCollection<IPullRequestModel>();
+            if (collection == null)
+                collection = new TrackingCollection<IPullRequestModel>();
 
             var source = Observable.Defer(() => keyobs
                 .SelectMany(key =>
@@ -156,7 +158,7 @@ public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryM
                         item =>
                         {
                             // this could blow up due to the collection being disposed somewhere else
-                            try { col.RemoveItem(Create(item)); }
+                            try { collection.RemoveItem(Create(item)); }
                             catch (ObjectDisposedException) { }
                         },
                         TimeSpan.FromMinutes(5),
@@ -165,8 +167,8 @@ public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryM
                 .Select(Create)
             );
 
-            col.Listen(source);
-            return col;
+            collection.Listen(source);
+            return collection;
         }
 
         public IObservable<Unit> InvalidateAll()

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

@@ -19,7 +19,8 @@ public interface IModelService : IDisposable
         IObservable<IReadOnlyList<IRepositoryModel>> GetRepositories();
         IObservable<IReadOnlyList<LicenseItem>> GetLicenses();
         IObservable<IReadOnlyList<GitIgnoreItem>> GetGitIgnoreTemplates();
-        ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryModel repo);
+        ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryModel repo,
+            ITrackingCollection<IPullRequestModel> collection = null);
         IObservable<Unit> InvalidateAll();
     }
 }