pull: Redo logic for "scanning"

pull: Redo logic for "scanning"

What in the code is called "scanning" is ensuring (potentially
recursively) have an object, and if not, fetching it.  And then if
it's metadata, parsing it and finding new objects to fetch.

This logic has grown fairly complex.  What I'm trying to fix
right now is that if we're doing a pull-local to a remote repository
via `sshfs` (FUSE) we still end up scanning, which is inefficient.

We can take advantage of the "commitpartial" logic here - if a commit
isn't partial, it's complete, hence we don't need to scan it.

At the same time, I'm changing the logic here to *always* do scans for
dirtree objects.  This will fix cases where multiple commits share
dirtree objects.  We have "commitpartial" metadata, but no such concept
of partial/complete for dirtrees.

But, we'll only ever scan dirtrees if we scan commits, which is
what the section above fixes.

Closes: https://github.com/ostreedev/ostree/issues/543
Closes: #564
Approved by: alexlarsson