narrow: make dirstateguard back up and restore working copy narrowspec instead

Authored by martinvonz.

Description

narrow: make dirstateguard back up and restore working copy narrowspec instead

We used to have only one narrowspec for the store and the working
copy, but now that we have one narrowspec for each, it seems clear
that the dirstateguard was supposed to back up and restore the
narrowspec associated with the working copy, not the one associated
with the store.

clearbackup() (for the store narrowspec) is not needed because the
presence of the file in localrepository._journalfiles() takes care of
that.

Differential Revision: https://phab.mercurial-scm.org/D5504