When threaded grep is in effect, the patterns are duplicated and
recompiled for each thread. Avoid "--debug" output during the
recompilation so that the output is given once instead of "1+nthreads"
times.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>