Code clean-up in the pathspec API.

* bw/pathspec-cleanup:
  pathspec: rename prefix_pathspec to init_pathspec_item
  pathspec: small readability changes
  pathspec: create strip submodule slash helpers
  pathspec: create parse_element_magic helper
  pathspec: create parse_long_magic function
  pathspec: create parse_short_magic function
  pathspec: factor global magic into its own function
  pathspec: simpler logic to prefix original pathspec elements
  pathspec: always show mnemonic and name in unsupported_magic
  pathspec: remove unused variable from unsupported_magic
  pathspec: copy and free owned memory
  pathspec: remove the deprecated get_pathspec function
  ls-tree: convert show_recursive to use the pathspec struct interface
  dir: convert fill_directory to use the pathspec struct interface
  dir: remove struct path_simplify
  mv: remove use of deprecated 'get_pathspec()'