diff --stat: allow custom diffstat output width.

This adds two parameters to "diff --stat".

 . --stat-width=72 tells that the page should fit on 72-column output.

 . --stat-name-width=30 tells that the filename part is limited
   to 30 columns.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -69,6 +69,9 @@ struct diff_options {
 	const char *stat_sep;
 	long xdl_opts;
 
+	int stat_width;
+	int stat_name_width;
+
 	int nr_paths;
 	const char **paths;
 	int *pathlens;