pull: do nothing on --dry-run

pull: do nothing on --dry-run

Pull was never meant to take --dry-run at all. However, it
passes unknown arguments to git-fetch, which does do a
dry-run. Unfortunately, pull then attempts to merge whatever
cruft was in FETCH_HEAD (which the dry-run fetch will not
have written to).

Even though we never advertise --dry-run as something that
should work, it is still worth being defensive because:

  1. Other commands (including fetch) take --dry-run, so a
     user might try it.

  2. Rather than simply producing an error, it actually
     changes the repository in totally unexpected ways.

This patch makes "pull --dry-run" equivalent to "fetch
--dry-run".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>