UseNewInterpField does not handle merge mode correctly

[wismill]

What I meant is the following:

static bool
UseNewInterpField(enum si_field field, SymInterpInfo *old, SymInterpInfo *new,
                  bool report, enum si_field *collide)
{
    if (!(old->defined & field))
        return true;

but, correctly for keys:

static bool
UseNewKeyField(enum key_field field, enum key_field old, enum key_field new,
               bool clobber, bool report, enum key_field *collide)
{
    if (!(old & field))
        return (new & field);

Example:

        interpret A {
            repeat = true;
        };
        interpret A {
            repeat = true;
        };
        augment interpret A {
            action = SetMods(mods=Mod1);
        };

Output:

	interpret A+AnyOfOrNone(all) {
		repeat= True;
		action= NoAction();
	};

Removing the second interpret produces the correct output:

	interpret A+AnyOfOrNone(all) {
		repeat= True;
		action= SetMods(modifiers=Mod1);
	};

Originally posted by @mahkoh in #565 (comment)