git-push: Accept -n as a synonym for --dry-run.

git-push is not currently using -n for anything else, and it seems
unlikely we will want to use it to mean anything else in the future,
so add it as an alias for convenience.

Signed-off-by: Nelson Elhage <nelhage@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>