cherry-pick: picking a tag that resolves to a commit is OK

Earlier, 21246dbb9e0a (cherry-pick: make sure all input objects are
commits, 2013-04-11) tried to catch an unlikely "git cherry-pick $blob"
as an error, but broke a more important use case to cherry-pick a
tag that points at a commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>