Please see the parent T1 for a description of the problem space.
Two separate considerations;
* With cloning the Cyrus IMAP repositories from their original locations over to our environment, pushes are not possible.
Without cloning the Cyrus IMAP repositories from their original locations over to our environment, the branching policy is arbitrary and our copy would be incomplete (this is what we currently do).
* Cyrus IMAP upstream also uses Phabricator, which would void auto-closing on branches that are upstream (i.e. a commit message with "Closes T4" cannot close T4).