admin: Add ot_admin_checksum_version to get a dup of the version for a commit
dump: Print the version when dumping a commit, log/show/etc.
admin status: Print the version
tests: Add some versioning metadata and test it appears in status output

Followup to previous commits.
checkout: permit checkout of a single file

fixes a coredump when using a command like:

$ ostree --repo=repo checkout -U --subpath=/usr/lib/passwd \
  fedora-atomic/rawhide/x86_64/docker-host usrlib-new

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>