Make fsck reachability avoid doing unnecessary work for parents that we reach multiple ways.
Make fsck reachability avoid doing unnecessary work for
parents that we reach multiple ways.

This doesn't matter right now. It _will_ matter once we have
complex revision graphs.

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git