merge --stat: use the full terminal width

Make merge --stat behave like diff --stat and use the full terminal
width.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -399,6 +399,7 @@ static void finish(struct commit *head_commit,
 	if (new_head && show_diffstat) {
 		struct diff_options opts;
 		diff_setup(&opts);
+		opts.stat_width = -1; /* use full terminal width */
 		opts.output_format |=
 			DIFF_FORMAT_SUMMARY | DIFF_FORMAT_DIFFSTAT;
 		opts.detect_rename = DIFF_DETECT_RENAME;