cpdup - Enhance -X operation

cpdup - Enhance -X operation

* Enhance -X operation.  When the cpignore file is specified as an
  absolute path, it may contain wildcards which match against the
  concatenated source path and directory element (as specified in the
  source), instead of just match against directory elements.

* Thus an absolute cpignore file may contain full paths and wildcards.
  For example, if you:

  cpdup -X $cwd/mycpignore /tmp /fubar

  And $cwd/mycpignore (specified with an absolute path) contains:

      /tmp/xyz/swap*

  Then cpdup will not copy files that match against /tmp/xyz/swap*.

  If you specify a relative path for the cpignore file, or use the
  -x option (instead of -X cpignorefile), operation remains as before
  and only matches against the directory element.