refs: allow @{n} to work with n-sized reflog

refs: allow @{n} to work with n-sized reflog

This sequence works

	$ git checkout -b newbranch
	$ git commit --allow-empty -m one
	$ git show -s newbranch@{1}

and shows the state that was immediately after the newbranch was
created.

But then if you do

	$ git reflog expire --expire=now refs/heads/newbranch
	$ git commit --allow=empty -m two
	$ git show -s newbranch@{1}

you'd be scolded with

	fatal: log for 'newbranch' only has 1 entries

While it is true that it has only 1 entry, we have enough
information in that single entry that records the transition between
the state in which the tip of the branch was pointing at commit
'one' to the new commit 'two' built on it, so we should be able to
answer "what object newbranch was pointing at?". But we refuse to
do so.

Make @{0} the special case where we use the new side to look up that
entry. Otherwise, look up @{n} using the old side of the (n-1)th entry
of the reflog.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>