Add and use cgit_find_stats_periodname() in print_repo()

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-stats.h

@@ -21,6 +21,7 @@ struct cgit_period {
 };
 
 extern int cgit_find_stats_period(const char *expr, struct cgit_period **period);
+extern const char *cgit_find_stats_periodname(int idx);
 
 extern void cgit_show_stats(struct cgit_context *ctx);