status: show progress bar if refreshing the index takes too long

status: show progress bar if refreshing the index takes too long

Refreshing the index is usually very fast, but it can still take a
long time sometimes. Cold cache is one. Or copying a repo to a new
place (*). It's good to show something to let the user know "git
status" is not hanging, it's just busy doing something.

(*) In this case, all stat info in the index becomes invalid and git
    falls back to rehashing all file content to see if there's any
    difference between updating stat info in the index. This is quite
    expensive. Even with a repo as small as git.git, it takes 3
    seconds.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>