read-tree: show progress by default

read-tree: show progress by default

The read-tree builtin has a --verbose option that signals to show
progress and other data while updating the index. Update this to
be on by default when stderr is a terminal window.

This will help tools like 'git sparse-checkout' to automatically
benefit from progress indicators when a user runs these commands.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>