git: Disable HAMMER live dedup, mark as experimental

[Thomas Nikolajsen]

>> Live dedup was suspected of data corruption (but not confirmed) --
>> the thought was to make sure people really, really mean to turn it on
>> for 3.0, it could be switched back to the way it was in master
>> now that 3.0 has been branched.
>
>Please switch back in master (& 3.0 IMO) &
>add a note to HAMMER man pages that live dedup is experimental.
>
>Please post a HEADS UP to users@
>otherwise we might still have users running live dedup
>(which didn't update to recent master).
>
>Also it would be nice to open a bug to track this issue.

Will do these things in a few days if I see no objections.

 -thomas