checkout: implement "@{-N}" shortcut name for N-th last branch

checkout: implement "@{-N}" shortcut name for N-th last branch

Implement a shortcut @{-N} for the N-th last branch checked out, that
works by parsing the reflog for the message added by previous
git-checkout invocations.  We expand the @{-N} to the branch name, so
that you end up on an attached HEAD on that branch.

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