A simple "git shortlog" outside of a git repository stalls
waiting for an input. Check if that's the case by testing with
isatty() before read_from_stdin(), and warn the user like
"git commit" does in a similar case.
Signed-off-by: Michele Ballabio <barra_cuda@katamail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>