Mercurial 4.1 release

1. Features

A new extensible "compression engines" API has been implemented. Support for a new compression format can now be implemented in extensions.
- New compression formats are fully supported in bundles and the wire protocol. There is experimental support for different compression in revlogs.
zstd - a new and exciting compression engine - is now distributed with Mercurial and built by default
- HTTP payloads for many commands will now use zstd by default (as opposed to zlib) when both client and server support it. This can reduce server-side CPU usage to ~60% of original for operations like serving bundles.
- zstd compressed bundles can be produced by specifying the zstd bundle type. e.g. hg bundle -a -t zstd-v2 zstd-bundle.hg.
A new statistical profiler has been added and made the default profiler for --profile. This profiler gives more accurate results than Python's built-in cProfile (the previous default) because the profiler overhead is much smaller. The output from --profile now displays the code hot path by default. This allows Mercurial developers to more accurately identify performance problems.
New followlines(file, from:to, startrev=.) revset predicate to track a part of changes.
Experimental support for additional git-diff features.
Troubled changes (from ChangesetEvolution) are noted more prominently in a few places.
An experimental hg debugupgraderepo command allows in-place upgrading of repositories to the latest storage format.

Performance of reading individual revlog entries has been greatly improved. This will be noticeable when performing changelog scans on large repositories (such as when executing certain revsets).
The low-level content diffing algorithm is now ~2x faster. Operations that write data to the repository (like commit) can be noticeably faster as a result.