git-show: Reject native ref

git-show: Reject native ref

So when we do

	git show v1.4.4..v1.5.0

that's an illogical thing to do, since "git show" is defined to be a
non-revision-walking action, which means the range operator be pointless
and wrong. The fact that we happily accept it (and then _only_ show
v1.5.0, which is the positive end of the range) is quite arguably not very
logical.

We should complain, and say that you can only do "no_walk" with positive
refs. Negative object refs really don't make any sense unless you walk
the obejct list (or you're "git diff" and know about ranges explicitly).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>