_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.
The previous diff algorithm was file tree based, and only looked
at modified files that lived at the same path.
However, components like the Linux kernel have versioned
subdirectories, e.g. /usr/lib/modules/$kver/.../ext4.ko. We want to
be able to detect these "modified renames" so that we can compute
diffs (rollsum, bsdiff).