t/t7407: demonstrate that the command called by 'submodule foreach' loses stdin

t/t7407: demonstrate that the command called by 'submodule foreach' loses stdin

The user-supplied command spawned by 'submodule foreach' loses its
connection to the original standard input.  Instead, it is connected to the
output of a pipe within the git-submodule script.  This can cause a problem
if the command requires reading from stdin or if it changes its behavior
based on whether stdin is a tty or not (e.g. git shortlog).  Demonstrate
this flaw.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>