man: git pull -r is a short for --rebase

man: git pull -r is a short for --rebase

Letting the "--rebase" option squat on the short-and-sweet single
letter option "-r" was an unintended accident and was not even
documented, but the short option seems to be already used in the
wild. Let's document it so that other options that begin with "r"
would not be tempted to steal it.

Signed-off-by: Miklos Vajna <vmiklos@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>