Fix: sync_back on new messages to preserve modifications to labels in before-add-message hook
Merge #299: sync message after before-add-hook has been run

runs the sync when we add a new message (i.e. not already in index) by
using the !m_old flag as the third argument to Index.sync_message. This
flushes the messages altered via the hook and resolves the
inconsistency.

Fixes #296.