Allow non-fast-forward of remote tracking branches in default clone

Allow non-fast-forward of remote tracking branches in default clone

This changes the default remote.origin.fetch configuration
created by git-clone so that it allows non-fast-forward updates.

When using the separate-remote layout with reflog enabled, it
does not make much sense to refuse to update the remote tracking
branch just because some of them do not fast-forward.  git-fetch
issues warnings on non-fast-forwardness, and the user can peek
at what the previous state was using the reflog.

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