checkout: no progress messages if !isatty(2).

checkout: no progress messages if !isatty(2).

If stderr isn't a tty, we shouldn't be printing incremental progress
messages.  In particular, this affects 'git checkout -f . >&logfile'
unless you provided -q.  And git-new-workdir has no way to provide -q.

It would probably be better to have progress.c check isatty(2) all the time,
but that wouldn't allow things like 'git push --progress' to force progress
reporting to on, so I won't try to solve the general case right now.

Actual fix suggested by Jeff King.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>