Open on GitHub/Copy Link to Clipboard fails when tracking branch has been deleted

[grokys]

When trying to run Open on GitHub/Copy Link to Clipboard on a branch whose tracking branch has been deleted, the following exception occurs:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=GitHub.Exports
  StackTrace:
   at GitHub.Services.GitService.<>c__DisplayClass6_0.<GetLatestPushedSha>b__0() in D:\projects\github\VisualStudio\src\GitHub.Exports\Services\GitService.cs:line 101
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()

This happens because repo.Head.TrackedBranch.Tip is null at:

https://github.com/github/VisualStudio/blob/master/src/GitHub.Exports/Services/GitService.cs?utf8=%E2%9C%93#L101

The commands fail silently.

[grokys]

Fixed by #1971.