grep: add --break

With --break, an empty line is printed between matches from different
files, increasing readability.  This option is taken from ack
(http://betterthangrep.com/).

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -110,6 +110,7 @@ struct grep_opt {
 	unsigned post_context;
 	unsigned last_shown;
 	int show_hunk_mark;
+	int file_break;
 	void *priv;
 
 	void (*output)(struct grep_opt *opt, const void *data, size_t size);