pickaxe: hoist empty needle check

pickaxe: hoist empty needle check

If we are given an empty pickaxe needle like "git log -S ''",
it is impossible for us to find anything (because no matter
what the content, the count will always be 0). We currently
check this at the lowest level of contains(). Let's hoist
the logic much earlier to has_changes(), so that it is
simpler to return our answer before loading any blob data.

Signed-off-by: Jeff King <peff@peff.net>