files: extract code for extra filtering of the `removed` entry into copies
We want to reduce the set of `removed` files that to the set of files actually
removed. That `removed` set is used as of the changeset centric algorithm,
having smaller sets means less processing and faster computation.
In this changeset we extract the code to be a function of it own. We will make
use of it in the next changesets.
Differential Revision: https://phab.mercurial-scm.org/D8588
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
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