Bug 1505579 - Upgrade to latest robustcheckout; r=sheehan

Vendored robustcheckout extension from the version-control-tools
repository revision d78e5c52a0228c3dc8ed03fe43bf749c8e68bd20 without
modifications.

This is needed to support Mercurial 4.8.

Differential Revision: https://phabricator.services.mozilla.com/D11398