check stderr with isatty() instead of stdout when deciding to show progress

check stderr with isatty() instead of stdout when deciding to show progress

Make transport code (viz. transport.c::fetch_refs_via_pack() and
transport-helper.c::standard_options()) that decides to show progress
check if stderr is a terminal, instead of stdout. After all, progress
reports (via the API in progress.[ch]) are sent to stderr.

Update the documentation for git-clone to say "standard error" as well.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>