Upgrade libgit2sharp

LibGit2Sharp cannot build pull-requests on private repos on V17 as it was unable to pass credentials in the CreateFakeBranchPointingAtThePullRequestTip method.

This is fixed in v18 which provides an option to pass credentials to Network.ListReferences
This PR upgrades LibGit2Sharp to enable this method to be used.

I'm not sure you're really willing to use `DefaultCredentials`. It's intended usage is to provide the "default" credentials (currently logged-in user information) via NTLM or SPNEGO authentication.

If you're rather after not providing any kind of credentials, just pass in `null`. That should work.