grep: show --debug output only once

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>

builtin/grep.c

@@ -209,6 +209,7 @@ static void start_threads(struct grep_opt *opt)
 		int err;
 		struct grep_opt *o = grep_opt_dup(opt);
 		o->output = strbuf_out;
+		o->debug = 0;
 		compile_grep_patterns(o);
 		err = pthread_create(&threads[i], NULL, run, o);