merge: allow "-" as a short-hand for "previous branch"

Just like "git checkout -" is a short-hand for "git checkout @{-1}" to
conveniently switch back to the previous branch, "git merge -" is a
short-hand for "git merge @{-1}" to conveniently merge the previous branch.

It will allow me to say:

    $ git checkout -b au/topic
    $ git am -s ./+au-topic.mbox
    $ git checkout pu
    $ git merge -

which is an extremely typical and repetitive operation during my git day.

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

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