Log Message:
jj: update to 0.21.0.

## [0.21.0] - 2024-09-04

### Breaking changes

* `next/prev` will no longer infer when to go into edit mode when moving from
  commit to commit. It now either follows the flags `--edit|--no-edit` or it
  gets the mode from `ui.movement.edit`.

### Deprecations

* `jj untrack` has been renamed to `jj file untrack`.

### New features

* Add new boolean config knob, `ui.movement.edit` for controlling the behaviour
  of `prev/next`. The flag turns `edit` mode `on` and `off` permanently when set
  respectively to `true` or `false`.

* All diff formats except `--name-only` now include information about copies and
  moves. So do external diff tools in file-by-file mode. `jj status` also
  includes information about copies and moves.

* Color-words diff has gained [an option to display complex changes as separate
  lines](docs/config.md#color-words-diff-options). It's enabled by default. To
  restore the old behavior, set `diff.color-words.max-inline-alternation = -1`.

* A tilde (`~`) at the start of the path will now be expanded to the user's home
  directory when configuring a `signing.key` for SSH commit signing.

* When reconfiguring the author, warn that the working copy won't be updated