Problem:
Old 'autoread' only did `:checktime` on focus-change and shell (":!")
commands, and only for non-hidden buffers. Since 'autoread' is now
driven by OS filewatcher events, buffers are updated much more eagerly.
This should be surfaced to the user somehow, either via a carefully
placed notification, or a minimal UI indicator.
A "notification" would be noisy, unless it is conditional on specific
circumstances (e.g. when "many" buffers are updated).
Solution:
Use the existing 'busy' buffer-local option as a subtle hint about
activity.
2abb9785d3
