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
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