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

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>