grep: allow threading even on a single-core machine

Earlier we disabled threading when online_cpus() said "1", but on a
filesystem with long latency (or in a cold cache situation), using
multiple threads to drive I/O in parallel would improve performance
even on a single-core machines.

Signed-off-by: Victor Leschuk <vleschuk@accesssoftek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -832,7 +832,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 #ifndef NO_PTHREADS
-	if (list.nr || cached || online_cpus() == 1)
+	if (list.nr || cached)
 		use_threads = 0;
 #else
 	use_threads = 0;