Fix t4201: accidental arithmetic expansion

instead of embedded subshell. It actually breaks here (dash as /bin/sh):

t4201-shortlog.sh: 27: Syntax error: Missing '))'
FATAL: Unexpected exit with code 2

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t4201-shortlog.sh

@@ -11,7 +11,7 @@ test_description='git-shortlog
 echo 1 > a1
 git add a1
 tree=$(git write-tree)
-commit=$((echo "Test"; echo) | git commit-tree $tree)
+commit=$( (echo "Test"; echo) | git commit-tree $tree )
 git update-ref HEAD $commit
 
 echo 2 > a1