Now that the computed similar objects are all regular files,
get_unpacked_unlinked_content should never be called on any other
object type. Assert that this is true instead of silently succeeding.
_ostree_delta_compute_similar_objects should not output symlinks.
Previously, a symlink in the "from" commit could be matched to a
real file in the "to" commit, since nothing was filtering symlinks
on the "from" side. This led to failures running the bzdiff
algorithm.
John Hiesey