narrow: only wrap dirstate functions once, instead of per-reposetup
chg will call reposetup multiple times, and we would end up double-wrapping (or
worse) the dirstate functions; this can cause issues like OSError 'No such file
or directory' during rebase operations, when we go to double-delete our
narrowspec backup file.
Differential Revision: https://phab.mercurial-scm.org/D3559