core: Add --tar-autocreate-parents option for commit

The tar files we're making of artifacts don't include parent
directories.  Now we could change the builder to make them, but we can
also just autocreate them on import.  Mode 0755 with no xattrs seems
OK here.

src/libostree/ostree-repo.h

@@ -158,6 +158,7 @@ gboolean      ostree_repo_stage_archive_to_mtree (OstreeRepo         *self,
                                                   GFile              *archive,
                                                   OstreeMutableTree  *tree,
                                                   OstreeRepoCommitModifier *modifier,
+                                                  gboolean            autocreate_parents,
                                                   GCancellable *cancellable,
                                                   GError      **error);