textconv: use shell to run helper

textconv: use shell to run helper

Currently textconv helpers are run directly. Running through
the shell is useful because the user can provide a program
with command line arguments, like "antiword -f".

It also makes textconv more consistent with other parts of
git, most of which run their helpers using the shell.

The downside is that textconv helpers with shell
metacharacters (like space) in the filename will be broken.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>