chat-store: fix errant 'this'
resolved a merge conflict, but retained a reference to 'this'
from the wrong parent.
?> ?=(%message -.act)
=/ mailbox=(unit mailbox) (~(get by inbox) path.act)
?~ mailbox
- [~ this]
+ [~ state]
=. letter.envelope.act (evaluate-letter [author letter]:envelope.act)
=. u.mailbox (append-envelope u.mailbox envelope.act)
:- (send-diff path.act act)
