refs: make repo_dwim_log() accept a NULL oid

refs: make repo_dwim_log() accept a NULL oid

Change the repo_dwim_log() function initially added as dwim_log() in
eb3a48221fd (log --reflog: use dwim_log, 2007-02-09) to accept a NULL
oid parameter. The refs_resolve_ref_unsafe() function it invokes
already deals with it, but it didn't.

This allows for a bit more clarity in a reflog-walk.c codepath added
in f2eba66d4d1 (Enable HEAD@{...} and make it independent from the
current branch, 2007-02-03). We'll shortly use this in
builtin/reflog.c as well.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>