Skip symlinked directories in Git::GatherDir

  Fixes RT#84491
v2.013

 - fix rt#84491 - Git::GatherDir does not handle symlinks
   Symlinks to directories used to make it crash with a cryptic error.
   It now skips them with a warning, and this is documented.
   (christopher madsen, reported by karen etheridge)
 - mention Git::GatherDir in Git (christopher madsen, reported by kevin dawson)