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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state