Use actual buffer contents for buffer previewers for some pickers
(buffers, treesitter, current_buffer_fuzzy_find, for starters). Not
to be confused with reusing buffers for preview buffers.

This lets users view previews of buffers with unwritten changes. For
treesitter and current_buffer_fuzzy_find, not showing actual buffer
content is arguably a bug considering the finder operates over the
buffer (written or not). So there are instances where you can search
over unwritten content in the buffer only to see an outdated, written
only content in the preview.