git-pickaxe: work properly in a subdirectory.

We forgot to add prefix to the given path.

[jc: interestingly enough, Jeff King had the same idea after I
pushed mine out to "pu", and his patch was cleaner, so I dropped
mine.]

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>