grep: refactor grep_objects loop into its own function

grep: refactor grep_objects loop into its own function

Simplify cmd_grep by splitting off the loop that finds matches in a
list of trees.  So now the main part of cmd_grep looks like:

	if (!use_index) {
		int hit = grep_directory(&opt, paths);
		if (use_threads)
			hit |= wait_all();
		return !hit;
	}
	if (!list.nr) {
		if (!cached)
			setup_work_tree();
		int hit = grep_cache(&opt, paths, cached);
		if (use_threads)
			hit |= wait_all;
		return !hit;
	}
	hit = grep_objects(&opt, path, &list);
	if (use_threads)
		hit |= wait_all();
	return !hit;

and is ripe for further refactoring.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>