Signal completion when opening repository using Get to Code

[jcansdale]

When the Get to Code flow calls AcquireCodeContainerAsync, it uses downloadProgress completion as a signal to close the dialog. This PR ensures that this signal is sent when opening a repository as well as cloning a repository.

What this PR does

• Adds tests for the GitHub.StartPage project / GitHubContainerProvider class
• Signal downloadProgress as complete before AcquireCodeContainerAsync returns a CodeContainer

It uses the following call to signal completion (1 of 1 steps complete with no progress message):

downloadProgress.Report(new ServiceProgressData(string.Empty, string.Empty, 1, 1));

How to test

1. Install a version of Visual Studio 2019 that includes Get to Code + GitHub integration
2. Open Visual Studio 2019
3. Install Dogfood VSIX + restart VS
4. Click Tools > Dogfood and select .vsix artifact (or URL) for this PR + restart VS
5. Click Clone or checkout code
6. Click on GitHub button under Browse a repository
7. Select a repository + local path that already exists
8. Click on Open button
9. Expect Clone or checkout code window to close
10. Expect your repository to be visible on Team Explorer tool window

Fixes #2211