git-verify-pack: insist on .idx extension

git-verify-pack: insist on .idx extension

git-verify-pack can be called with a filename without .idx extension.
add_packed_git() on the other hand depends on its presence.  So
instead of trying to call it with whatever the user gave us check for
that extension and add it if it's missing.

That means that you can't name your index file "blah" and your pack
file ".pack" anymore ("git-verify-pack blah" currently works in that
case).  I think this regression is a good change. ;-)

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>