changing-files: add clean computation of changed files for roots
The `files` field is not reliable, so we need to compute things from scratch. We
start with the simplest case root changesets. In the beginning they was nothing,
then user said "let there be files" and there were added files.
Differential Revision: https://phab.mercurial-scm.org/D9126
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
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
instructions, platform-specific notes, and Mercurial user information.