git-submodule.sh: preserve stdin for the command spawned by foreach

git-submodule.sh: preserve stdin for the command spawned by foreach

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.  The user-supplied
command supplied to 'submodule foreach' is spawned within a while loop
which is being piped into.  Due to the way shells implement piping output
to a while loop, a subshell is created with its standard input attached to
the output of the pipe.  This results in all of the commands executed
within the while loop to have their stdins modified in the same way,
including the user-supplied command.

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.  For
example, this problem was noticed when trying to execute the following:

   git submodule foreach git shortlog --since=two.weeks.ago

which printed a message about entering the first submodule and produced no
further output and exited with a status of zero.  In this case, shortlog
detected that it was not connected to a tty, and since no revision was
supplied as an argument, it attempted to read the list of revisions from
standard input.  Instead, it slurped up the list of submodules that was
being piped to the enclosing while loop and caused that loop to end early
without processing the remaining submodules.

Work around this behavior by saving the original standard input file
descriptor before the while loop, and restoring it when spawning the
user-supplied command.

This fixes the tests in t7407.

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