narrow: only diff manifest part within narrowspec when generating changegroup
Since
959ebff3505a (manifest: add match argument to diff and
filesnotin, 2017-03-07), we have a more efficient way of diffing
manifests while applying a matcher. Let's use that while generating
narrowed changegroups, so we avoid diffing parts of the manifest that
don't match the narrowspec.
Differential Revision: https://phab.mercurial-scm.org/D2233