ref-filter.h: provide REF_FILTER_INIT

Provide a sane initialization value for `struct ref_filter`, which in a
subsequent patch will be used to initialize a new field.

In the meantime, ensure that the `ref_filter` struct used in the
test-helper's `cmd__reach()` is zero-initialized. The lack of
initialization is OK, since `commit_contains()` only looks at the single
`with_commit_tag_algo` field that *is* initialized directly above.

So this does not fix a bug, but rather prevents one from biting us in
the future.

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

ref-filter.h

@@ -92,6 +92,9 @@ struct ref_format {
 	struct string_list bases;
 };
 
+#define REF_FILTER_INIT { \
+	.points_at = OID_ARRAY_INIT, \
+}
 #define REF_FORMAT_INIT {             \
 	.use_color = -1,              \
 	.bases = STRING_LIST_INIT_DUP, \