narrow: add --no-backup option for narrowing Most of our users at Google use Mercurial on a file system that keeps backups of previous versions of all files, including those in `.hg/`. They therefore don't need a separate backup in the file system when narrowing their repo (which they typically do by running `hg tracked --auto-remove-includes`). Backups can be very slow. `hg strip` already has a `--no-backup` option. This patch adds the same option to `hg tracked --removeinclude/--addexclude`. Differential Revision: https://phab.mercurial-scm.org/D9951