tests: add newfilenode test case in test-merge-changedelete.t

rust: leverage improved match ergonomics Differential Revision: https://phab.mercurial-scm.org/D9137

rust: update minimum supported Rust compiler version Debian stable backported `rustc 1.41.1` for a Firefox release that needed it, so we can finally update the compiler version to something that isn't more than a year old. This means that (among other things [1]), `async await` syntax is usable! Differential Revision: https://phab.mercurial-scm.org/D9138

changing-files: add a utility to compute the merged files post-commit We will need it in `_getsidedata` as soon as we start persisting that set. Differential Revision: https://phab.mercurial-scm.org/D9089

changing-files: record merged files at commit time The data is easy to gather at commit time, and we need it for changeset centric copy tracing. Right now, it is not persisted so we cannot use it. However we will fix this part very soon, gathering something to persist was necessary first. Differential Revision: https://phab.mercurial-scm.org/D9088

changing-files: add the ability to track merged files too The set of merged files is used when doing changeset centric copy tracing (cf `is_merged` in `mercurial/copies.py`. So tracking (and persisting) this set will be useful. We start with adding the attribute on the new object. Differential Revision: https://phab.mercurial-scm.org/D9087

rust: start plugging the dirstate tree behind a feature gate The previous patch added the `dirstate-tree` feature gate to enable the two dirstate implementations to co-habit while the tree-based one gets better. This patch copies over the code that differs, be it because the algorithm changed or because the borrowing rules are different. Indeed, `DirstateTree` is not observationally equivalent to the std `HashMap` in the APIs we use: it does not have the `Entry` API (yet?) and its iterator returns owned values instead of references. This last point is because the implementation needs to be changed to a more clever and efficient solution. Differential Revision: https://phab.mercurial-scm.org/D9133

rust: introduce `dirstate-tree` cargo feature This feature gates (at compile-time) the use of the newly-added dirstate tree. The motivation for this is that the dirstate tree is currently *very* slow; replacing the current hashmap-based dirstate is not a viable solution in terms of performance... and why would you be using the Rust implementation if not for performance? The feature will also help reviewers better understand the differences that will slowly appear as the dirstate tree gets better. Differential Revision: https://phab.mercurial-scm.org/D9132

rust: fix formatting Maybe there is a small divergence in the nightly formatter? This didn't seem to trip the CI. Differential Revision: https://phab.mercurial-scm.org/D9131

rust-matchers: make `Matcher` trait object-safe Before this patch, it is not possible to create a `Matcher` trait-object (like `Box<dyn Matcher>`), because of the use of a generic parameters in some methods, namely `impl AsRef<HgPath>`. While this makes the interface less flexible for callers in theory, it does not change anything in the current codebase. Until something like [1] is implemented, this is a "tradeoff" that we need to make anyway. [1] https://internals.rust-lang.org/t/pre-rfc-expand-object-safety/12693 Differential Revision: https://phab.mercurial-scm.org/D9071

rust: clippy pass This removes some justified warnings and one hard error that, while technically not a bug, was an ugly oversight on my part. Differential Revision: https://phab.mercurial-scm.org/D9094

updaterepo: add deprecation warning We just moved all in-tree callers off of the function, but we're leaving it here for a bit to help extension writers. Differential Revision: https://phab.mercurial-scm.org/D9110

rhg: use `.or(Err(Error))` not `.map_err(|_| Error)` (D9100#inline-15067) Differential Revision: https://phab.mercurial-scm.org/D9109

hg-core: use `.or(Err(Error))` not `.map_err(|_| Error)` (D9100#inline-15067) Differential Revision: https://phab.mercurial-scm.org/D9108

hg-core: return Err if `offset != bytes.len()` (D8958#inline-14994 followup 2/2) [X] make `Index` owner of its bytes [X] make `Index::new` return an error if `offset != bytes.len()` Differential Revision: https://phab.mercurial-scm.org/D9107