core: Improve checkout API

Expose the lower-level functionality in libostree, change checkout
builtin to be a higher level driver.  This will allow us to more
easily improve the "checkout" builtin..

src/libostree/ostree-repo.h

@@ -205,12 +205,14 @@ typedef enum {
   OSTREE_REPO_CHECKOUT_MODE_USER
 } OstreeRepoCheckoutMode;
 
-gboolean      ostree_repo_checkout (OstreeRepo              *self,
-                                    OstreeRepoCheckoutMode   mode,
-                                    const char              *ref,
-                                    GFile                   *destination,
-                                    GCancellable            *cancellable,
-                                    GError                 **error);
+gboolean
+ostree_repo_checkout_tree (OstreeRepo               *self,
+                           OstreeRepoCheckoutMode    mode,
+                           GFile                    *destination,
+                           OstreeRepoFile           *source,
+                           GFileInfo                *source_info,
+                           GCancellable             *cancellable,
+                           GError                  **error);
 
 gboolean       ostree_repo_read_commit (OstreeRepo *self,
                                         const char *rev,