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 <[email protected]>
Signed-off-by: Johan Herland <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jherland

t/t7407-submodule-foreach.sh

@@ -201,14 +201,16 @@ submodulesha1=$(cd clone3/nested1/nested2/nested3/submodule && git rev-parse HEA
 sub1sha1=$(cd clone3/sub1 && git rev-parse HEAD)
 sub2sha1=$(cd clone3/sub2 && git rev-parse HEAD)
 sub3sha1=$(cd clone3/sub3 && git rev-parse HEAD)
+sub1sha1_short=$(cd clone3/sub1 && git rev-parse --short HEAD)
+sub2sha1_short=$(cd clone3/sub2 && git rev-parse --short HEAD)
 
 cat > expect <<EOF
  $nested1sha1 nested1 (heads/master)
  $nested2sha1 nested1/nested2 (heads/master)
  $nested3sha1 nested1/nested2/nested3 (heads/master)
  $submodulesha1 nested1/nested2/nested3/submodule (heads/master)
- $sub1sha1 sub1 (${sub1sha1:0:7})
- $sub2sha1 sub2 (${sub1sha1:0:7})
+ $sub1sha1 sub1 ($sub1sha1_short)
+ $sub2sha1 sub2 ($sub2sha1_short)
  $sub3sha1 sub3 (heads/master)
 EOF