Convert cgit_print_error to a variadic function

This removes many uses of "fmt" which uses a fixed size static pool of
fixed size buffers.  Instead of relying on these, we now pass around
argument lists for as long as possible before using a strbuf to render
content of an arbitrary size.

Signed-off-by: John Keeping <john@keeping.me.uk>

ui-shared.h

@@ -52,7 +52,10 @@ extern void cgit_object_link(struct object *obj);
 extern void cgit_submodule_link(const char *class, char *path,
 				const char *rev);
 
-extern void cgit_print_error(const char *msg);
+__attribute__((format (printf,1,2)))
+extern void cgit_print_error(const char *fmt, ...);
+__attribute__((format (printf,1,0)))
+extern void cgit_vprint_error(const char *fmt, va_list ap);
 extern void cgit_print_date(time_t secs, const char *format, int local_time);
 extern void cgit_print_age(time_t t, time_t max_relative, const char *format);
 extern void cgit_print_http_headers(struct cgit_context *ctx);