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

src/libostree/ostree-repo.h

@@ -362,6 +362,12 @@ gboolean      ostree_repo_abort_transaction (OstreeRepo     *self,
                                              GCancellable   *cancellable,
                                              GError        **error);
 
+_OSTREE_PUBLIC
+gboolean      ostree_repo_mark_commit_partial (OstreeRepo     *self,
+                                               const char     *checksum,
+                                               gboolean        is_partial,
+                                               GError        **error);
+
 _OSTREE_PUBLIC
 void          ostree_repo_transaction_set_refspec (OstreeRepo *self,
                                                    const char *refspec,