Merge branch 'mh/ceiling' into next

Merge branch 'mh/ceiling' into next

Elements of GIT_CEILING_DIRECTORIES list may not match the real
pathname we obtain from getcwd(), leading the GIT_DIR discovery
logic to escape the ceilings the user thought to have specified.

* mh/ceiling:
  string_list_longest_prefix(): remove function
  setup_git_directory_gently_1(): resolve symlinks in ceiling paths
  longest_ancestor_length(): require prefix list entries to be normalized
  longest_ancestor_length(): take a string_list argument for prefixes
  longest_ancestor_length(): use string_list_split()
  Introduce new function real_path_if_valid()
  real_path_internal(): add comment explaining use of cwd
  Introduce new static function real_path_internal()