narrow: extract repo property for store narrowmatcher
When a repo lock is released, we try to persist the manifest
cache. That involves getting the narrowmatcher for the
manifestlog. That should not fail if the store and working copy
narrowspecs are out of sync. Without this patch, the later patches in
this series will fail because of that.
Differential Revision: https://phab.mercurial-scm.org/D5508