filter-branch: fix dash complaining about "Missing '))'"

On e.g. Ubuntu, dash is used as /bin/sh.  Unlike bash it parses
commands like

  a=$((echo stuff) | wc)

as an arithmetic expression while what we want is a subshell inside
a command substitution.  Resolve the ambiguity by placing a space
between the two opening parentheses.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -298,7 +298,7 @@ do
 	# Assign the boundarie(s) in the set of rewritten commits
 	# as the replacement commit(s).
 	# (This would look a bit nicer if --not --stdin worked.)
-	for p in $((cd "$workdir"/../map; ls | sed "s/^/^/") |
+	for p in $( (cd "$workdir"/../map; ls | sed "s/^/^/") |
 		git rev-list $ref --boundary --stdin |
 		sed -n "s/^-//p")
 	do