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jj is a Git-compatible version control system that is both simple and powerful. See the installation instructions to get started.

The experimental support from release 0.29.0 for transferring the change ID to/from Git remotes has been enabled by default. The change ID is stored in the Git commit itself (in a commit header called change-id), which means it will be transferred by regular git push etc. Please let us know if you run into any problems with it. You can disable it setting git.write-change-id-header. Note that some Git remotes (e.g GitLab) and some Git commands (e.g. git rebase) do not preserve the change ids when they rewrite commits.
jj rebase now automatically abandons divergent commits if another commit with the same change ID is already present in the destination with identical changes.
jj split has gained --message, --insert-before, --insert-after, and --destination options.
jj evolog can show associated operations for commits created by new jj versions.

The old libgit2 code path for fetches and pushes has been removed, and the git.subprocess setting along with it.
In templates, bookmark/tag/remote names are now formatted in revset symbol notation. The type of bookmark.remote() is changed to Option<_>. bookmark.remote() == "foo" still works, but bookmark.remote().<method>() might need if(bookmark.remote(), ..) to suppress error.
jj rebase now automatically abandons divergent commits if another commit with the same change ID is already present in the destination with identical changes. To keep these divergent commits, use the --keep-divergent flag.
The deprecated --skip-empty flag for jj rebase has been removed. Use the --skip-emptied flag instead.
The deprecated jj branch subcommands have been removed. Use the jj bookmark subcommands instead.
jj util completion now requires the name of the shell as a positional argument and no longer produces Bash completions by default. The deprecated optional arguments for different shells have been removed.
(Update: this entry was mistakenly omitted in the release notes previously) External diff tools are now run in the temporary directory containing the before (left) and after (right) directories, making diffs appear more pleasing for tools that display file paths prominently. Users can opt out of this by setting merge-tools.<tool>.diff-do-chdir = false, but this will likely be removed in a future release. Please report any issues you run into.

The ui.diff.format and ui.diff.tool config options have been merged as ui.diff-formatter. The builtin format can be specified as :<format> (e.g. ui.diff-formatter=":git" for Git diffs.)
The .normal_hex() method will be removed from the CommitId template type. It's useful only for the ChangeId type.

jj split has gained a --message option to set the description of the commit with the selected changes.
jj split has gained the ability to place the revision with the selected changes anywhere in the revision tree with the --insert-before, --insert-after and --destination command line flags.
Added git.track-default-bookmark-on-clone setting to control whether to track the default remote bookmark on jj git clone.
Templates can now do arithmetic on integers with the +, -, *, /, and % infix operators.
Evolution history is now stored in the operation log. jj evolog can show associated operations for commits created by new jj versions.

Work around a git issue that could cause subprocess operations to hang if the core.fsmonitor gitconfig is set in the global or system gitconfigs. #6440
jj parallelize can now parallelize groups of changes that start with an immutable change, but do not contain any other immutable changes.
jj will no longer warn about deprecated paths on macOS if the configured XDG directory is the deprecated one (~/Library/Application Support).
The builtin diff editor now correctly handles splitting changes where a file is replaced by a directory of the same name. #5189

Due to the removal of the libgit2 code path, packagers should remove any dependencies on libgit2, libssh2, Zlib, OpenSSL, and pkg-config, and ensure they are not setting the Cargo git2 or vendored-openssl features.

Thanks to the people who made this release happen!