use __attribute__ to catch printf format mistakes

Use "__attribute__((format (printf,N,M)))", as is done in git, do catch
mistakes in printf-style format strings.

Signed-off-by: Mark Lodato <lodatom@gmail.com>

cache.h

@@ -30,6 +30,7 @@ extern int cache_process(int size, const char *path, const char *key, int ttl,
 extern int cache_ls(const char *path);
 
 /* Print a message to stdout */
+__attribute__((format (printf,1,2)))
 extern void cache_log(const char *format, ...);
 
 extern unsigned long hash_str(const char *str);

cgit.h

@@ -293,6 +293,7 @@ extern void cgit_diff_tree(const unsigned char *old_sha1,
 
 extern void cgit_diff_commit(struct commit *commit, filepair_fn fn);
 
+__attribute__((format (printf,1,2)))
 extern char *fmt(const char *format,...);
 
 extern struct commitinfo *cgit_parse_commit(struct commit *commit);

html.h

@@ -5,7 +5,10 @@ extern int htmlfd;
 
 extern void html_raw(const char *txt, size_t size);
 extern void html(const char *txt);
+
+__attribute__((format (printf,1,2)))
 extern void htmlf(const char *format,...);
+
 extern void html_status(int code, const char *msg, int more_headers);
 extern void html_txt(const char *txt);
 extern void html_ntxt(int len, const char *txt);