sparse: fix pax extraction for unicode filenames

sparse: fix pax extraction for unicode filenames

Make sure that 'GNU.sparse.name' header has higher priority than
(for sparse-purposes artificially modified) 'path' pax header.

Historically, the 'GNU.sparse.name' header comes before 'path';
this caused that modified 'path' header won and that is not what
we want in sparse "capable" tar implementation.

* src/tar.h (tar_stat_info): New argument sparse_name_done.
* src/xheader.c (raw_path_decoder): Move here the unconditional
code from path_decoder.
(path_decoder): Apply raw_path_decoder only if sparse_path_decoder
was not yet called.
(sparse_path_decoder): New wrapper around raw_path_decoder.
* tests/sparse07.at: New testcase.
* tests/testsuite.at: Mention new testcase.
* tests/Makefile.am: Likewise.