Allow users to un-configure rename detection

Allow users to un-configure rename detection

I told people on the kernel mailing list to please use "-M" when sending
me rename patches, so that I can see what they do while reading email
rather than having to apply the patch and then look at the end result.

I also told them that if they want to make it the default, they can just
add

	[diff]
		renames

to their ~/.gitconfig file. And while I was thinking about that, I wanted
to also check whether you can then mark individual projects to _not_ have
that default in the per-repository .git/config file.

And you can't. Currently you cannot have a global "enable renames by
default" and then a local ".. but not for _this_ project". Why? Because if
somebody writes

	[diff]
		renames = no

we simply ignore it, rather than resetting "diff_detect_rename_default"
back to zero.

Fixed thusly.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>