Fails to check out PR branch.

[grokys]

Version

• GitHub Extension for Visual Studio version: 2.5.7.5090
• Visual Studio version: 15.8.7

What happened

Steps to Reproduce

1. Clone https://github.com/AvaloniaUI/Avalonia
2. Open Support for d:DataContext, d:DesignWidth and d:DesignHeight AvaloniaUI/Avalonia#1952 in GitHub pane
3. Click "Checkout" link

Expected behavior: [What you expect to happen]

PR branch is checked out

Actual behavior: [What actually happens]

Log file:

2018-10-10 22:53:41.150 [39972] EROR [06] GitClient                 Failed to fetch
LibGit2Sharp.LibGit2SharpException: Request failed with status code: 404
   at LibGit2Sharp.Core.Ensure.HandleError(Int32 result)
   at LibGit2Sharp.Core.Proxy.git_remote_fetch(RemoteHandle remote, IEnumerable`1 refSpecs, GitFetchOptions fetchOptions, String logMessage)
   at LibGit2Sharp.Commands.Fetch(Repository repository, String remote, IEnumerable`1 refspecs, FetchOptions options, String logMessage)
   at LibGit2Sharp.Network.Fetch(Remote remote, FetchOptions options)
   at GitHub.Services.GitClient.<>c__DisplayClass9_0.<Fetch>b__0()

[grokys]

I think this is caused by the fact that the fork that submitted the PR has a different name.

[jcansdale]

There is possibly another edge case where a user submits a PR and then deletes their fork. What happens to PRs when their downstream repository is deleted?