Allow >1 PR to be created :)

Don't call `OnCompleted` on `create` as more than one PR can be created
from this view.

This together with the previous commit fixes #325.

Author: grokys

src/GitHub.VisualStudio/UI/Views/PullRequestListView.xaml.cs

@@ -56,7 +56,6 @@ protected void NotifyOpen(ViewWithData id)
         protected void NotifyCreate()
         {
             create.OnNext(null);
-            create.OnCompleted();
         }
 
         bool disposed;