git-pickaxe: do not confuse two origins that are the same.

git-pickaxe: do not confuse two origins that are the same.

It used to be that we can compare the address of the origin
structure to determine if they are the same because they are
always registered with scoreboard.  After introduction of the
loop to try finding the best split, that is not true anymore.

The current code has rather serious leaks with origin structure,
but more importantly it gets confused when two origins that
points at the same commit and same path.

We might eventually have to refcount and gc origin, but let's
fix the correctness issue first.

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