When passed no revision arguments, ‘git shortlog’ reads a log from
stdin if and only if stdin is not a tty.  So scripts that need to
function identically when standard input is a terminal (as when run
interactively) and not (as when run through a cron job) should either
supply a log themselves or specify the desired revisions explicitly.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>