make git-shell paranoid about closed stdin/stdout/stderr

It is in general unsafe to start a program with one or more of file
descriptors 0/1/2 closed.  Karl Chen for example noticed that stat_command
does this in order to rename a pipe file descriptor to 0:

    dup2(from, 0);
    close(from);

... but if stdin was closed (for example) from == 0, so that

    dup2(0, 0);
    close(0);

just ends up closing the pipe.  Another extremely rare but nasty problem
would occur if an "important" file ends up in file descriptor 2, and is
corrupted by a call to die().

Fixing this in git was considered to be overkill, so this patch works
around it only for git-shell.  The fix is simply to open all the "low"
descriptors to /dev/null in main.

Signed-off-by: Paolo Bonzini <bonzini@gnu.org>
Acked-by: Stephen R. van den Berg <srb@cuci.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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