git-checkout: fix command line parsing.

git-checkout: fix command line parsing.

This fixes an issue when you use:

    $ git checkout -- <path1> [<paths>...]

and that <path1> can also be understood as a reference. git-checkout
mistakenly understands this as the same as:

    $ git checkout <path1> -- [<paths>...]

because parse-options was eating the '--' and the argument parser thought
he was parsing:

    $ git checkout <path1> [<paths>...]

Where there indeed is an ambiguity

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>