OstreeMutableTree: Refactor: Add `parent` pointer

OstreeMutableTree: Refactor: Add `parent` pointer

This implements a TODO item from
`ostree_mutable_tree_get_contents_checksum`.  We now no-longer invalidate
the dirtree contents checksum at `get_contents_checksum` time - we
invalidate it when the mtree is modified.  This is implemented by keeping
a pointer to the parent directory in each `OstreeMutableTree`.  This gives
us stronger invariants on `contents_checksum`.

For even stronger guarantees about invariants we could make
`ostree_repo_write_mtree` or similar a member of `OstreeMutableTree` and
remove `ostree_mutable_tree_set_metadata_checksum`.

I think I've fixed a bug here too.  We now invalidate parent's contents
checksum when our metadata checksum changes, whereas we didn't before.

Closes: #1655
Approved by: cgwalters