core: Add API (and standard concept for) content checksum

core: Add API (and standard concept for) content checksum

There are a few cases for knowing whether a commit has identical
content to another commit.  Some people want to do a "promotion workflow",
where the content of a commit on a tesitng branch is then "promoted"
to a production branch with `ostree commit --tree=ref`.

Another use case I just hit in rpm-ostree deals with
[jigdo](https://github.com/projectatomic/rpm-ostree/issues/1081) where we're
importing RPMs on both the client and server, and will be using the
content checksum, since the client/server cases inject different metadata
into the commit object.

Closes: https://github.com/ostreedev/ostree/issues/1315
Closes: #1449
Approved by: jlebon