core: Check out asynchronously

This can be a large performance win in certain circumstances:

 * Cold buffer cache (we don't block the whole process)
 * Requiring a copy instead of hardlink

src/libostree/ostree-repo.h

@@ -307,15 +307,21 @@ typedef enum {
   OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES = 1
 } OstreeRepoCheckoutOverwriteMode;
 
+void
+ostree_repo_checkout_tree_async (OstreeRepo               *self,
+                                 OstreeRepoCheckoutMode    mode,
+                                 OstreeRepoCheckoutOverwriteMode    overwrite_mode,
+                                 GFile                    *destination,
+                                 OstreeRepoFile           *source,
+                                 GFileInfo                *source_info,
+                                 GCancellable             *cancellable,
+                                 GAsyncReadyCallback       callback,
+                                 gpointer                  user_data);
+
 gboolean
-ostree_repo_checkout_tree (OstreeRepo               *self,
-                           OstreeRepoCheckoutMode    mode,
-                           OstreeRepoCheckoutOverwriteMode    overwrite_mode,
-                           GFile                    *destination,
-                           OstreeRepoFile           *source,
-                           GFileInfo                *source_info,
-                           GCancellable             *cancellable,
-                           GError                  **error);
+ostree_repo_checkout_tree_finish (OstreeRepo               *self,
+                                  GAsyncResult             *result,
+                                  GError                  **error);
 
 gboolean       ostree_repo_read_commit (OstreeRepo *self,
                                         const char *rev,