Fix git-rev-parse over-eager errors

Using "--verify" together with "--no-flags" makes perfect sense, but
git-rev-parse would complain about it when it saw a flag, even though it
would never actually use/output that flag.

This fixes it.

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