libostree: mark ostree_repo_checkout_tree_at as deprecated

and move its definition to a separate file.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #417
Approved by: cgwalters

src/libostree/ostree-repo-checkout.c

@@ -844,6 +844,8 @@ ostree_repo_checkout_tree (OstreeRepo               *self,
  * Note in addition that unlike ostree_repo_checkout_tree(), the
  * default is not to use the repository-internal uncompressed objects
  * cache.
+ *
+ * This function is deprecated.  Use ostree_repo_checkout_at() instead.
  */
 gboolean
 ostree_repo_checkout_tree_at (OstreeRepo                        *self,