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-prune.c

@@ -36,16 +36,6 @@ typedef struct {
   guint64 freed_bytes;
 } OtPruneData;
 
-static gboolean
-prune_commitpartial_file (OstreeRepo    *repo,
-                          const char    *checksum,
-                          GCancellable  *cancellable,
-                          GError       **error)
-{
-  g_autofree char *path = _ostree_get_commitpartial_path (checksum);
-  return ot_ensure_unlinked_at (repo->repo_dir_fd, path, error);
-}
-
 static gboolean
 maybe_prune_loose_object (OtPruneData        *data,
                           OstreeRepoPruneFlags    flags,
@@ -68,7 +58,7 @@ maybe_prune_loose_object (OtPruneData        *data,
 
           if (objtype == OSTREE_OBJECT_TYPE_COMMIT)
             {
-              if (!prune_commitpartial_file (data->repo, checksum, cancellable, error))
+              if (!ostree_repo_mark_commit_partial (data->repo, checksum, FALSE, error))
                 return FALSE;
             }