pack-object: squelch eye-candy on non-tty

pack-object: squelch eye-candy on non-tty

One of my post-update scripts runs a git-fetch into a separate
repository and sends the results back to me (2>&1); I end up
getting this in the mail:

    Generating pack...
    Done counting 180 objects.
    Result has 131 objects.
    Deltifying 131 objects.
       0% (0/131) done^M   1% (2/131) done^M...

This defaults not to do the progress report when not on a tty.

You could give --progress to force the progress report, but
let's not bother even documenting it nor mentioning it in the
usage string.

Signed-off-by: Junio C Hamano <junkio@cox.net>