narrow: call narrowspec.{save,restore,clear}backup directly I want to move .hg/narrowspec to .hg/store/narrowspec and we need to decouple the narrowspec update from the dirstate update for that. This patch lets the callers call the narrowspec backup functions directly, in addition to the dirstate backup functions. The narrowspec methods are made to check if narrowing is enabled. For that, a repo instance was needed, which all the callers luckily already had available. Differential Revision: https://phab.mercurial-scm.org/D4096