Description:
------------
The `Phar::buildFromDirectory()` method generates tar archives in `PAX` format, so different tar archives that contain the same files may have different `sha1` checksums.

Test script:
---------------
<?php
  // mkdir test && echo 'hello' > test/test.txt
  $phar = new \PharData('a.tar');
  $phar->buildFromDirectory('test');
  // sleep for 1 second to produce different atime.
  sleep(1);
  $phar = new \PharData('b.tar');
  $phar->buildFromDirectory('test');
  echo sha1_file('a.tar') . "\n";                                                                                          
  echo sha1_file('b.tar') . "\n";
  // a.tar and b.tar contain the same files, but have different checksums
  // Looks like the `buildFromDirectory()` method generates tar archives
  // with `PAX` headers. Calling "tar --format=ustar -cf ..." from command line
  // resolves this issue.

Expected result:
----------------
The sha1 checksums for a.tar and b.tar should be the same.

Actual result:
--------------
The sha1 checksums for a.tar and b.tar are different.