rhg: Add support for `rhg status --copies`
Copy sources are collected during `status()` rather than after the fact like
in Python, because `status()` takes a `&mut` exclusive reference to the dirstate map
(in order to potentially mutate it for directory mtimes) and returns `Cow<'_, HgPath>`
that borrow the dirstate map.
Even though with `Cow` only some shared borrows remain, the still extend the same
lifetime of the initial `&mut` so the dirstate map cannot be borrowed again
to access copy sources after the fact:
https://doc.rust-lang.org/nomicon/lifetime-mismatch.html#limits-of-lifetimes
Additionally, collecting copy sources during the dirstate tree traversal that
`status()` already does avoids the cost of another traversal or other lookups
(though I haven’t benchmarked that cost).
Differential Revision: https://phab.mercurial-scm.org/D11899
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors