core: Add --link-cache option to checkout

This is a convenient way to have a lookaside directory of hard links,
which can greatly speed up checkouts.  In the future we probably want
to push this down into the repository.

src/libostree/ostree-repo.h

@@ -310,6 +310,7 @@ gboolean
 ostree_repo_checkout_tree (OstreeRepo               *self,
                            OstreeRepoCheckoutMode    mode,
                            OstreeRepoCheckoutOverwriteMode    overwrite_mode,
+                           GFile                    *link_cache,
                            GFile                    *destination,
                            OstreeRepoFile           *source,
                            GFileInfo                *source_info,