scalar: show progress if stderr refers to a terminal
Sometimes when users use scalar to download a monorepo with a long
commit history, they want to check the progress bar to know how long
they still need to wait during the fetch process, but scalar
suppresses this output by default.
So let's check whether scalar stderr refer to a terminal, if so,
show progress, otherwise disable it.
Signed-off-by: ZheNing Hu <adlternative@gmail.com> Acked-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>