request-pull: protect against OPTIONS_KEEPDASHDASH from environment

request-pull: protect against OPTIONS_KEEPDASHDASH from environment

Like most git commands, request-pull supports a -- delimiter to allow
callers to pass arguments that would otherwise be treated as an option
afterwards.  The internal OPTIONS_KEEPDASHDASH variable is passed
empty to git-sh-setup to indicate that request-pull itself does not
care about the position of the -- delimiter.  But if the user has
that variable in her environment, request-pull will see the “--” and
fail.

Empty it explicitly to guard against this.  While at it, make the
corresponding fix to git-resurrect, too (all other scripts in git.git
already protect themselves).

Acked-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>