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commitctx: document the manifest writing function

Same spirit as for `_filecommit` lets document the input and output before
making any change or improvement.

This is part of a larger refactoring/cleanup of the commitctx code to clarify
and augment the logic gathering metadata useful for copy tracing. The current
code is a tad too long and entangled to make such update easy. We start with
easy and small cleanup.

Differential Revision: https://phab.mercurial-scm.org/D8705

== 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.