This teaches pack-objects to use .gitattributes mechanism so
that the user can specify certain blobs are not worth spending
CPU cycles to attempt deltification.

The name of the attrbute is "delta", and when it is set to
false, like this:

	== .gitattributes ==
	*.jpg	-delta

they are always stored in the plain-compressed base object
representation.
Signed-off-by: NJunio C Hamano <junkio@cox.net>