Documentation/shortlog: scripted users should not rely on implicit HEAD

Documentation/shortlog: scripted users should not rely on implicit HEAD

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: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>