reftable: mark unused parameters in virtual functions

The reftable code uses a lot of virtual function pointers, but many of
the concrete implementations do not need all of the parameters.

For the most part these are obviously fine to just mark as UNUSED (e.g.,
the empty_iterator functions unsurprisingly do not do anything). Here
are a few cases where I dug a little deeper (but still ended up just
marking them UNUSED):

  - the iterator exclude_patterns is best-effort and optional (though it
    would be nice to support in the long run as an optimization)

  - ignoring the ref_store in many transaction functions is unexpected,
    but works because the ref_transaction itself carries enough
    information to do what we need.

  - ignoring "err" for in some cases (e.g., transaction abort) is OK
    because we do not return any errors. It is a little odd for
    reftable_be_create_reflog(), though, since we do return errors
    there. We should perhaps be creating string error messages at this
    layer, but I've punted on that for now.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-reftable-merged.c

@@ -22,7 +22,7 @@ static ssize_t strbuf_add_void(void *b, const void *data, const size_t sz)
 	return sz;
 }
 
-static int noop_flush(void *arg)
+static int noop_flush(void *arg UNUSED)
 {
 	return 0;
 }