commit: avoid writing to global in option callback

The callback function for --trailer writes directly to the global
trailer_args and ignores opt->value completely. This is OK, since that's
where we expect to find the value. But it does mean the option
declaration isn't as clear. E.g., we have:

    OPT_BOOL(0, "reset-author", &renew_authorship, ...),
    OPT_CALLBACK_F(0, "trailer", NULL, ..., opt_pass_trailer)

In the first one we can see where the result will be stored, but in the
second, we get only NULL, and you have to go read the callback.

Let's pass &trailer_args, and use it in the callback. As a bonus, this
silences a -Wunused-parameter warning.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

builtin/commit.c

@@ -139,7 +139,7 @@ static int opt_pass_trailer(const struct option *opt, const char *arg, int unset
 {
 	BUG_ON_OPT_NEG(unset);
 
-	strvec_pushl(&trailer_args, "--trailer", arg, NULL);
+	strvec_pushl(opt->value, "--trailer", arg, NULL);
 	return 0;
 }
 
@@ -1633,7 +1633,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "fixup", &fixup_message, N_("[(amend|reword):]commit"), N_("use autosquash formatted message to fixup or amend/reword specified commit")),
 		OPT_STRING(0, "squash", &squash_message, N_("commit"), N_("use autosquash formatted message to squash specified commit")),
 		OPT_BOOL(0, "reset-author", &renew_authorship, N_("the commit is authored by me now (used with -C/-c/--amend)")),
-		OPT_CALLBACK_F(0, "trailer", NULL, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, opt_pass_trailer),
+		OPT_CALLBACK_F(0, "trailer", &trailer_args, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, opt_pass_trailer),
 		OPT_BOOL('s', "signoff", &signoff, N_("add a Signed-off-by trailer")),
 		OPT_FILENAME('t', "template", &template_file, N_("use specified template file")),
 		OPT_BOOL('e', "edit", &edit_flag, N_("force edit of commit")),