repo: Add a "sysroot-path" property

Adds ostree_repo_new_for_sysroot_path() to specify a system root path.
OstreeRepo otherwise uses _ostree_get_default_sysroot_path().

src/libostree/ostree-repo-private.h

@@ -54,6 +54,7 @@ struct OstreeRepo {
   GFile *uncompressed_objects_dir;
   int uncompressed_objects_dir_fd;
   GFile *config_file;
+  GFile *sysroot_dir;
 
   GFile *transaction_lock_path;
   GHashTable *txn_refs;