Adjust stat width calculations to take --graph output into account

The recent change to compute the width of diff --stat did not take into
consideration the output from --graph. The consequence is that when both
options are used, e.g. in 'log --stat --graph', the lines are too long.

Adjust stat width calculations to take --graph output into account.

Signed-off-by: Lucian Poston <lucian.poston@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -1445,7 +1445,7 @@ static void show_stats(struct diffstat_t *data, struct diff_options *options)
 	 */
 
 	if (options->stat_width == -1)
-		width = term_columns();
+		width = term_columns() - options->output_prefix_length;
 	else
 		width = options->stat_width ? options->stat_width : 80;