tests/test-gendoc-da.t

shelve: also create class representing whole directory of shelves

It's a little annoying to have to create and pass in a vfs into
`listshelves()`. This patch attempts to start addressing that by
creating a class that represents a directory containing shelves (the
directory can be either `.hg/shelved/` or `.hg/shelve-backup/`).

Differential Revision: https://phab.mercurial-scm.org/D9743

#require docutils gettext

  $ $TESTDIR/check-gendoc da
  checking for parse errors