upload-pack: squelch progress indicator if client cannot see it

upload-pack: squelch progress indicator if client cannot see it

upload-pack runs pack-objects, which generates progress indicator output
on its stderr. If the client requests a sideband, this indicator is sent
to the client; but if it did not, then the progress is written to
upload-pack's own stderr.

If upload-pack is itself run from git-daemon (and if the client did not
request a sideband) the progress indicator never reaches the client and it
need not be generated in the first place. With this patch the progress
indicator is suppressed in this situation.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>