reflog-walk: skip over double-null oid due to HEAD rename

reflog-walk: skip over double-null oid due to HEAD rename

Since 39ee4c6c2f (branch: record creation of renamed branch
in HEAD's log, 2017-02-20), a rename on the currently
checked out branch will create two entries in the HEAD
reflog: one where the branch goes away (switching to the
null oid), and one where it comes back (switching away from
the null oid).

This confuses the reflog-walk code. When walking backwards,
it first sees the null oid in the "old" field of the second
entry. Thanks to the "root commit" logic added by 71abeb753f
(reflog: continue walking the reflog past root commits,
2016-06-03), we keep looking for the next entry by scanning
the "new" field from the previous entry. But that field is
also null! We need to go just a tiny bit further, and look
at its "old" field. But with the current code, we decide the
reflog has nothing else to show and just give up. To the
user this looks like the reflog was truncated by the rename
operation, when in fact those entries are still there.

This patch does the absolute minimal fix, which is to look
back that one extra level and keep traversing.

The resulting behavior may not be the _best_ thing to do in
the long run (for example, we show both reflog entries each
with the same commit id), but it's a simple way to fix the
problem without risking further regressions.

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