t7407: Use 'rev-parse --short' rather than bash's substring expansion notation

t7407: Use 'rev-parse --short' rather than bash's substring expansion notation

The substring expansion notation is a bashism that we have not so far
adopted.  Use 'git rev-parse --short' instead, as this also handles
the case where the unique abbreviation is longer than 7 characters.

Also fix the typo; the object name for submodule #2 was copied from
submodule #1's by mistake.

Suggested-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>