relnotes/next

discovery: fix docstring of `outgoing` class

Also, introduce a more correct name `ancestorsof` for what was named
`missingheads` before. For now, we just forward `ancestorsof` to `missingheads`
until all users are changed.

There were some mistakes in the old docstring / name:

* `missingheads` (new name: `ancestorsof`) contains the revs whose ancestors
  are included in the outgoing operation. It may contain non-head revs and revs
  which are already on the remote, so the name "missingheads" is wrong in two
  ways.
* `missing` contains only ancestors of `missingheads`, so not *all nodes*
  present in local but not in remote.
* `common` might not contain all common revs, e.g. not some that are not an
  ancestor of `missingheads`.

It seems like the misleading name have fostered an actual bug (issue6372),
where `outgoing.missingheads` was used assuming that it contains the heads of
the missing changesets.

== New Features ==

 * clonebundles can be annotated with the expected memory requirements
   using the `REQUIREDRAM` option. This allows clients to skip
   bundles created with large zstd windows and fallback to larger, but
   less demanding bundles.

== New Experimental Features ==

 * The core of some hg operations have been (and are being)
   implemented in rust, for speed. `hg status` on a repository with
   300k tracked files goes from 1.8s to 0.6s for instance.
   This has currently been tested only on linux, and does not build on
   windows. See rust/README.rst in the mercurial repository for
   instructions to opt into this.

== Backwards Compatibility Changes ==

* Mercurial now requires at least Python 2.7.9 or a Python version that
  backported modern SSL/TLS features (as defined in PEP 466), and that Python
  was compiled against a OpenSSL version supporting TLS 1.1 or TLS 1.2
  (likely this requires the OpenSSL version to be at least 1.0.1).

* The `hg perfwrite` command from contrib/perf.py was made more flexible and
  changed its default behavior. To get the previous behavior, run `hg perfwrite
  --nlines=100000 --nitems=1 --item='Testing write performance' --batch-line`.


== Internal API Changes ==

 * logcmdutil.diffordiffstat() now takes contexts instead of nodes.

 * The `mergestate` class along with some related methods and constants have
   moved from `mercurial.merge` to a new `mercurial.mergestate` module.