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>

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);