Issue Type: Bug

On Windows where Git it slow, I frequently see that the repositories property on the Git extension API is first initialized to an empty array, and then later an update event is sent with the actual repositories. When consuming this API from the GitHub Pull Requests extension, this causes the tree text to read "No GitHub repositories found" on startup before the event is fired and the tree is loaded for whatever remotes are found. It would be great if we could avoid showing this intermediate and confusing text to users by having the repositories property set from the beginning.

VS Code version: Code - Insiders 1.37.0-insider (81ed55a, 2019-07-22T07:16:47.743Z)
OS version: Windows_NT x64 10.0.18362