diff --no-index: die on error reading stdin

diff --no-index: die on error reading stdin

If there is an error when reading from stdin then "diff --no-index"
prints an error message but continues to try and diff a file named "-"
resulting in an error message that looks like

    error: error while reading from stdin: Invalid argument
    fatal: stat '-': No such file or directory

assuming that no file named "-" exists. If such a file exists it prints
the first error message and generates the diff from that file which is
not what the user wanted. Instead just die() straight away if we cannot
read from stdin.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>