add rest api for content categories

I think the only duplicates we'd want to be aware of is on UUID at this point.
Also this raises probably another important issue, git has the concept of file paths to help identify things that "the same", possibly this is included in the `sha` checksum that git uses to identify changesets.

Should we be looking at that stuff alongside UUIDs? Are those our UUIDs?

What've you found so far with your experiments in libgit that points at that?