rhg: make output of `files` relative to the current directory and the root
This matches the behavior of `hg files`.
The util is added in `hg-core` instead of `rhg` because this operation could
be useful for other external tools. (this was definitely not prompted by rust
issue #50784, I swear)
Differential Revision: https://phab.mercurial-scm.org/D8872
{
"version": 1,
"project": "mercurial",
"project_url": "https://mercurial-scm.org/",
"repo": "..",
"branches": ["default", "stable"],
"environment_type": "virtualenv",
"show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
"benchmark_dir": "benchmarks",
"env_dir": "../.asv/env",
"results_dir": "../.asv/results",
"html_dir": "../.asv/html"
}