interpret-trailers: mark unused "unset" parameters in option callbacks

There are a few parse-option callbacks that do not look at their "unset"
parameters, but also do not set PARSE_OPT_NONEG. At first glance this
seems like a bug, as we'd ignore "--no-if-exists", etc.

But they do work fine, because when "unset" is true, then "arg" is NULL.
And all three functions pass "arg" on to helper functions which do the
right thing with the NULL.

Note that this shortcut would not be correct if any callback used
PARSE_OPT_NOARG (in which case "arg" would be NULL but "unset" would be
false). But none of these do.

So the code is fine as-is. But we'll want to mark the unused "unset"
parameters to quiet -Wunused-parameter. I've also added a comment to
make this rather subtle situation more explicit.

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

Author: peff

builtin/interpret-trailers.c

@@ -24,20 +24,23 @@ static enum trailer_if_exists if_exists;
 static enum trailer_if_missing if_missing;
 
 static int option_parse_where(const struct option *opt,
-			      const char *arg, int unset)
+			      const char *arg, int unset UNUSED)
 {
+	/* unset implies NULL arg, which is handled in our helper */
 	return trailer_set_where(opt->value, arg);
 }
 
 static int option_parse_if_exists(const struct option *opt,
-				  const char *arg, int unset)
+				  const char *arg, int unset UNUSED)
 {
+	/* unset implies NULL arg, which is handled in our helper */
 	return trailer_set_if_exists(opt->value, arg);
 }
 
 static int option_parse_if_missing(const struct option *opt,
-				   const char *arg, int unset)
+				   const char *arg, int unset UNUSED)
 {
+	/* unset implies NULL arg, which is handled in our helper */
 	return trailer_set_if_missing(opt->value, arg);
 }