revision: clarify a 'return NULL' in get_reference()

revision: clarify a 'return NULL' in get_reference()

When we know a pointer variable is NULL, it's clearer to
explicitly return NULL than to return that variable.

In get_reference(), when 'object' is NULL, we already return NULL
when 'revs->exclude_promisor_objects && is_promisor_object(oid)' is
true, but we return 'object' when 'revs->ignore_missing' is true.

Let's make the code clearer and more uniform by also explicitly
returning NULL when 'revs->ignore_missing' is true.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>