The CommitSource was used to specify the source URL and checksum of a
commit for use in manifest sources. Renaming to CommitSpec and adding a
Ref parameter generalises the type so that we can use it to specify
commits in various situations. This is building towards separating when
ostree parameters are used for fetching a commit, fetching a parent
commit, and building one.
The CommitSpec is (very roughly) analogous to the rpmmd.PackageSpec.
