git-gpull: Pass arguments after -- to git-pull

git-gpull's help text suggests that unrecognized arguments are passed to
git-pull:

    Usage:
        git gpull [<git-pull options>]

Before this patch, that wasn't the case. Unrecognized arguments yielded
an error.

Now, we pass all arguments after the first encountered -- to git-pull.
This is useful for running git-gpull in a dirty repository with
git-pull's --autostash argument.

Change-Id: Ia3b6f53e39aea4ba5fb6a1c2ba965ed73c737430
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>