split: use literal syntax to build a set of one element

rust-cpython: leverage py_shared_iterator::from_inner() where appropriate

rust-cpython: remove Option<_> from interface of py_shared_iterator It's the implementation detail of the py_shared_iterator that the leaked reference is kept in Option<_> so that it can be dropped early.

rust-cpython: rename py_shared_iterator_impl to py_shared_iterator It's a public interface now.

rust-cpython: replace dyn Iterator<..> of mapping with concrete type See the previous commit for why. The docstring is moved accordingly.

rust-cpython: replace dyn Iterator<..> of sequence with concrete type We wouldn't care the cost of the dynamic dispatch, but I feel a concrete type helps understanding error messages.

rust-dirstate: provide CopyMapIter and StateMapIter types They will be used in the declaration of Python iterator types.

rust-dirstate: specify concrete return type of DirsMultiset::iter() This allows us to put a returned iterator in a struct. We could implement DirsMultisetIter(hash_map::Keys<..>) struct to hide the implementation detail, but I think type alias is good enough for us.

discovery: replace "heads" by "changesets" in a output note (BC) When using `hg push --rev X`, the subset considered by discovery is only `::X`. In addition, `X` can be any local revisions not just local heads. As a result the message "all local heads known locally" can be misleading. We replace it with the more accurate "all local changesets known remotely". The message appears when in verbose not, so this is stricly speaking a BC breakage. I am not sure this would be a real issue in practice...

py3: drop incorrect fsencode(findexe(...)) in procutil I recently added the bad call, thinking that findexe() was a standard library function returning a string result, but it's actually our own function returning bytes. Thanks to Yuya for noticing. Differential Revision: https://phab.mercurial-scm.org/D6826