format-patch: fix parsing of "--" on the command line

format-patch: fix parsing of "--" on the command line

When given a pathspec that does not match any path in the current work
tree with an explicit "--":

    git format-patch <commit> -- <path>

the command still complains that <path> does not exist in the current work
tree and the user needs to explicitly specify "--" and errors out.  This
is because it incorrectly removes "--" from the command line arguments
that is later passed to setup_revisions().

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>