lib/repo: Add an API to mark a commit as partial

For the [rpm-ostree jigdo ♲📦](https://github.com/projectatomic/rpm-ostree/issues/1081) work.
We're basically doing "pull" via a non-libostree mechanism, and this
should be fully supported.  As I mentioned earlier we should try to
have `ostree-repo-pull.c` only use public APIs; this gets us closer
to that.

Closes: #1376
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -320,6 +320,7 @@ ostree_repo_set_alias_ref_immediate
 ostree_repo_set_cache_dir
 ostree_repo_sign_delta
 ostree_repo_has_object
+ostree_repo_mark_commit_partial
 ostree_repo_write_metadata
 ostree_repo_write_metadata_async
 ostree_repo_write_metadata_finish