Move deny_non_fast_forwards handling completely into receive-pack.

Move deny_non_fast_forwards handling completely into receive-pack.

The 'receive.denynonfastforwards' option has nothing to do with
the repository format version.  Since receive-pack already uses
git_config to initialize itself before executing any updates we
can use the normal configuration strategy and isolate the receive
specific variables away from the core variables.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>