diff.c: mention completion above add_diff_options

Add a comment on top of add_diff_options, where common diff options are
listed, mentioning __git_diff_common_options in the completion script,
in the hope that contributors update it when they add new diff flags.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -5491,6 +5491,10 @@ static int diff_opt_rotate_to(const struct option *opt, const char *arg, int uns
 	return 0;
 }
 
+/*
+ * Consider adding new flags to __git_diff_common_options
+ * in contrib/completion/git-completion.bash
+ */
 struct option *add_diff_options(const struct option *opts,
 				struct diff_options *options)
 {