v0.20.0

### Note to packagers

* `jj` now links `libgit2` statically by default. To use dynamic linking, you
  need to set the environment variable `LIBGIT2_NO_VENDOR=1` while compiling.
  ([#4163](https://github.com/martinvonz/jj/pull/4163))

### Breaking changes

* `jj rebase --skip-empty` has been renamed to `jj rebase --skip-emptied`

* `jj backout --revision` has been renamed to `jj backout --revisions`.
  The short alias `-r` is still supported.

* [The default `immutable_heads()` set](docs/config.md#set-of-immutable-commits)
  now includes `untracked_remote_branches()` with the assumption that untracked
  branches aren't managed by you. Therefore, untracked branches are no longer
  displayed in `jj log` by default.

* Updated defaults for graph node symbol templates `templates.log_node` and
  `templates.op_log_node`.

* [The "fileset" language](docs/filesets.md) is now enabled by default. It can
  still be disable by setting `ui.allow-filesets=false`.

* On `jj git fetch`/`import`, commits referred to by `HEAD@git` are no longer
  preserved. If a checked-out named branch gets deleted locally or remotely, the
  corresponding commits will be abandoned.

* `jj --at-op=@` no longer merges concurrent operations if explicitly specified.

* `jj obslog -p` no longer shows diffs at non-partial squash operations.
  Previously, it showed the same diffs as the second predecessor.

### Deprecations

* The original configuration syntax for `jj fix` is now deprecated in favor of
  one that allows defining multiple tools that can affect different filesets.
  These can be used in combination for now. See `jj help fix` for details.

### New features

* External diff tools can now be configured to invoke the tool on each file
  individually instead of being passed a directory by setting
  `merge-tools.$TOOL.diff-invocation-mode="file-by-file"` in config.toml.

* In git diffs, word-level hunks are now highlighted with underline. See [diff
  colors and styles](docs/config.md#diff-colors-and-styles) for customization.

* New `.diff().<format>()` commit template methods are added. They can be used
  in order to show diffs conditionally. For example,
  `if(current_working_copy, diff.summary())`.

* `jj git clone` and `jj git init` with an existing git repository adds the
  default branch of the remote as repository settings for
  `revset-aliases."trunk()"`.`

* `jj workspace forget` now abandons the workspace's working-copy commit if it
  was empty.

* `jj backout` now includes the backed out commit's subject in the new commit
  message.

* `jj backout` can now back out multiple commits at once.

* `jj git clone some/nested/path` now creates the full directory tree for 
   nested destination paths if they don't exist.

* String patterns now support case‐insensitive matching by suffixing any
  pattern kind with `-i`. `mine()` uses case‐insensitive matching on your email
  address unconditionally. Only ASCII case folding is currently implemented,
  but this will likely change in the future.

* String patterns now support `regex:"pattern"`.

* New `tracked_remote_branches()` and `untracked_remote_branches()` revset
  functions can be used to select tracked/untracked remote branches.

* The `file()` revset function now accepts fileset as argument.

* New `diff_contains()` revset function can be used to search diffs.

* New command `jj operation diff` that can compare changes made between two
  operations.

* New command `jj operation show` that can show the changes made in a single
  operation.

* New config setting `git.private-commits` to prevent commits from being pushed.

* [The default commit description template](docs/config.md#default-description)
  can now be configured by `templates.draft_commit_description`.

* `jj fix` can now be configured to run different tools on different filesets.
  This simplifies the use case of configuring code formatters for specific file
  types. See `jj help fix` for details.

* Added revset functions `author_date` and `committer_date`.

* `jj describe` can now update the description of multiple commits.

### Fixed bugs

* `jj status` will show different messages in a conflicted tree, depending
  on the state of the working commit. In particular, if a child commit fixes
  a conflict in the parent, this will be reflected in the hint provided
  by `jj status`

* `jj diff --git` no longer shows the contents of binary files.

* Windows binaries no longer require `vcruntime140.dll` to be installed
  (normally through Visual Studio.)

* On quit, the builtin pager no longer waits for all outputs to be discarded.

* `jj branch rename` no longer shows a warning in colocated repos.

### Contributors

Thanks to the people who made this release happen!

* Anton Älgmyr (@algmyr)
* Austin Seipp (@thoughtpolice)
* Benjamin Tan (@bnjmnt4n)
* Daniel Ploch (@torquestomp)
* Danny Hooper (@hooper)
* Emily (@emilazy)
* Essien Ita Essien (@essiene)
* Erich Gubler (@ErichDonGubler)
* Fedor Sheremetyev (@sheremetyev)
* Ilya Grigoriev (@ilyagr)
* Jonathan Tan (@jonathantanmy)
* Julien Vincent (@julienvincent)
* Martin von Zweigbergk (@martinvonz)
* Matt Kulukundis (@fowles)
* Matt Stark (@matts1)
* mlcui (@mlcui-corp)
* Philip Metzger (@PhilipMetzger)
* Scott Taylor (@scott2000)
* Skyler Grey (@Minion3665)
* Stephen Jennings (@jennings)
* Tim Janik (@tim-janik)
* Vincent Ging Ho Yim (@cenviity)
* Vladimír Čunát (@vcunat)
* Vladimir (@0xdeafbeef)
* Yuya Nishihara (@yuja)