Log Message:
jj: update to 0.12.0.

## [0.12.0] - 2023-12-05

### Breaking changes

* The `remote_branches()` revset no longer includes branches exported to the Git
  repository (so called Git-tracking branches.)

* `jj branch set` no longer creates a new branch. Use `jj branch create`
  instead.

* `jj init --git` in an existing Git repository now errors and exits rather than
  creating a second Git store.

### New features

* `jj workspace add` can now take _multiple_ `--revision` arguments, which will
  create a new workspace with its working-copy commit on top of all the parents,
  as if you had run `jj new r1 r2 r3 ...`.

* You can now set `git.abandon-unreachable-commits = false` to disable the
  usual behavior where commits that became unreachable in the Git repo are
  abandoned ([#2504](https://github.com/martinvonz/jj/pull/2504)).

* `jj new` gained a `--no-edit` option to prevent editing the newly created
  commit. For example, `jj new a b --no-edit -m Merge` creates a merge commit
  without affecting the working copy.

* `jj rebase` now takes the flag `--skip-empty`, which doesn't copy over commits
  that would become empty after a rebase.

* There is a new `jj util gc` command for cleaning up the repository storage.
  For now, it simply runs `git gc` on the backing Git repo (when using the Git
  backend).

### Fixed bugs

* Fixed another file conflict resolution issue where `jj status` would disagree
  with the actual file content.
  [#2654](https://github.com/martinvonz/jj/issues/2654)