- Mercurial 4.1
This is a regularly-scheduled quarterly feature release.

+ 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.
+ Improvements