about: always ensure page has a trailing slash

Otherwise we can't easily embed links to other /about/ pages.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cmd.h

@@ -2,10 +2,12 @@
 #define CMD_H
 
 typedef void (*cgit_cmd_fn)(void);
+typedef void (*cgit_cmd_pre_fn)(void);
 
 struct cgit_cmd {
 	const char *name;
 	cgit_cmd_fn fn;
+	cgit_cmd_pre_fn pre;
 	unsigned int want_repo:1,
 		want_layout:1,
 		want_vpath:1,