don't refetch all chat history, download diff only

reading up the existing json chat history ( if exist ) could mean that fetching of chat messages can be cut down to only new messages.

Start by locating the code that reads existing JSON chat history and fetches Keybase chat messages; no specific files or tests are named in the issue. Compare stored messages with newly fetched messages, and consider the work complete when an existing backup fetches only messages not already recorded without losing chat history.

Written by the indexing model from the issue text.