metaedit: switch to scmutil.cleanupnodes for cleanup

Summary:
That switches us to use the higher level abstractions and allows us to
beneefit from things like showupdated (see: D173)

Test Plan: ran metaedit tests

Reviewers: #fbhgext, quark

Reviewed By: #fbhgext, quark

Subscribers: quark

Differential Revision: https://phab.mercurial-scm.org/D191