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

src/libostree/ostree-core.h

@@ -520,6 +520,9 @@ gchar *  ostree_commit_get_parent            (GVariant  *commit_variant);
 _OSTREE_PUBLIC
 guint64  ostree_commit_get_timestamp         (GVariant  *commit_variant);
 
+_OSTREE_PUBLIC
+gchar *  ostree_commit_get_content_checksum  (GVariant  *commit_variant);
+
 _OSTREE_PUBLIC
 gboolean ostree_check_version (guint required_year, guint required_release);