fix bsd shell negation

fix bsd shell negation

On some shells (notably /bin/sh on FreeBSD 6.1), the
construct

  foo && ! bar | baz

is true if

  foo && baz

whereas for most other shells (such as bash) is true if

  foo && ! baz

We can work around this by specifying

  foo && ! (bar | baz)

which works everywhere.

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