[PATCH] Allow "+remote:local" refspec to cause --force when fetching.

With this we could say:

Pull: master:ko-master +pu:ko-pu

to mean "fast forward ko-master with master, overwrite ko-pu with pu",
and the latter one does not require the remote "pu" to be descendant
of local "ko-pu".

Signed-off-by: Junio C Hamano <junkio@cox.net>