narrow: don't resurrect old commits when narrowing (don't strip obsmarkers)

Authored by martinvonz.

Description

narrow: don't resurrect old commits when narrowing (don't strip obsmarkers)

If you have an old obsolescence-chain of commits that has been pruned
and you narrow your repo so that some of those commits get stripped
(because they affected the removed paths), then we would currently
resurrect the commit that came before (along the obsmarker chain) the
last stripped commit. That happens by the usual rules for
obsmarker-stripping. However, it's quite surprising when it happens
when you narrow your repo. This patch makes narrowing not strip
obsmarkers.

Differential Revision: https://phab.mercurial-scm.org/D5364