-Even with shallow clone, storing the history of large/busy mailing lists
-may place much burden on subscribers and servers. However, having a
-single (or few) refs representing the entire history of a list is good
-for small lists since it's easier to lookup a message by Message-ID, so
-we want to avoid splitting refs with independent histories.
-
-ssoma will likely grow its own builtin ref rotation system based on
-message count (not rotating at fixed time intervals). This would
-split the histories and require O(n) lookup time based on Message-ID,
-where `n' is the number of history splits.