pack-objects: split implications of --all-progress from progress activation

pack-objects: split implications of --all-progress from progress activation

Currently the --all-progress flag is used to use force progress display
during the writing object phase even if output goes to stdout which is
primarily the case during a push operation.  This has the unfortunate
side effect of forcing progress display even if stderr is not a
terminal.

Let's introduce the --all-progress-implied argument which has the same
intent except for actually forcing the activation of any progress
display.  With this, progress display will be automatically inhibited
whenever stderr is not a terminal, or full progress display will be
included otherwise.  This should let people use 'git push' within a cron
job without filling their logs with useless percentage displays.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Tested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>