t5704: fix nonportable sed/grep usages

OS X's sed and grep would complain with (respectively)

  sed: 1: "/^-/{p;q}": extra characters at the end of q command
  grep: Regular expression too big

For sed, use an explicit ; to terminate the q command.

For grep, spell the "40 hex digits" explicitly in the regex, which
should be safe as other tests already use this and we haven't got
breakage reports on OS X about them.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>