Downloader hangs when executed with no items

[StanleyGoldman]

Might be related to: #841

Depends on:

• Disabling BranchListTest in AppVeyor #879

Testing local builds, where the conditions for downloading resources are slightly different.
I found an issue where the Downloader will hang when RunSynchronously is called with no items in the queue.

Unity/src/GitHub.Api/Installer/GitInstaller.cs

Lines 260 to 264 in 675714a

	if (!state.GitZipExists && !state.GitIsValid && state.GitPackage != null)
	downloader.QueueDownload(state.GitPackage.Uri, installDetails.ZipPath);
	if (!state.GitLfsZipExists && !state.GitLfsIsValid && state.GitLfsPackage != null)
	downloader.QueueDownload(state.GitLfsPackage.Uri, installDetails.ZipPath);
	downloader.RunSynchronously();

It hangs because TrySetResult/TrySetException is never called.