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
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