*.c static functions: add missing __attribute__((format))

Add missing __attribute__((format)) function attributes to various
"static" functions that take printf arguments.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -2408,6 +2408,7 @@ cleanup:
 #define VERIFY_COMMIT_GRAPH_ERROR_HASH 2
 static int verify_commit_graph_error;
 
+__attribute__((format (printf, 1, 2)))
 static void graph_report(const char *fmt, ...)
 {
 	va_list ap;