filter-branch: use $(($i+1)) instead of $((i+1))

The expression $((i+1)) is not portable at all: even some bash versions
do not grok it. So do not use it.

Noticed by Jonas Fonseca.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -339,7 +339,7 @@ test $commits -eq 0 && die "Found nothing to rewrite"
 
 i=0
 while read commit; do
-	i=$((i+1))
+	i=$(($i+1))
 	printf "$commit ($i/$commits) "
 
 	git-read-tree -i -m $commit