narrow: reuse narrowspec.updateworkingcopy() when widening
The widening of the working copy we do after widening a repo is
practically the same as we do in a repo share after the store
narrowspec has been changed in a different share. Let's reuse the code
for that that we now have in the narrowspec module.
Differential Revision: https://phab.mercurial-scm.org/D5510