`jj parallelize` was a good example of a command that can be
simplified by the new API, so I decided to rewrite it as an example.
The rewritten version is more flexible and doesn't actually need the
restrictions from the old version (such as checking that the commits
are connected). I still left the check for now to keep this patch
somewhat small. A subsequent commit will remove the restrictions.
Parallelize revisions by making them siblings
Running `jj parallelize 1::2` will transform the history like this:
```text
3
| 3
2 / \
| -> 1 2
1 \ /
| 0
0
```
Each of the target revisions is rebased onto the parents of the root(s) of
the target revset (not to be confused with the repo root). The children of
the head(s) of the target revset are rebased onto the target revisions.
The target revset is the union of the REVISIONS arguments.
The target revset being parallelized must satisfy several conditions,
otherwise the command will fail.
1. The heads of the target revset must not have different children.
2. The roots of the target revset must not have different parents.
3. The parents of all target revisions except the roots must also be
parallelized. This means that the target revisions must be connected.
