FR: Include an option to jj new and jj rebase to remove redundant parents

[samueltardieu]

Is your feature request related to a problem? Please describe.
On some projects, I use a "super-merge" model, where a dev-merge bookmark is based on the project main branch + all my PR waiting to be reviewed and merged + some third-party PR of interests that I want to integrate as well.

I rebase this bookmark every time I start working, and because some PR may have been merged at this stage, in addition to the main branch advancing, I have to systematically use jj simplify-parents -s dev-merge after I use jj rebase -s dev-merge -d all:dev-merge- -d main in order to clean the tree.

Also, sometimes, I want to try a few third-party PRs and see how they interact with the current main branch. Here also, after doing a jj new -d main -d pr1@origin1 -d pr2@origin2, I do a jj simplify-parents.

Describe the solution you'd like
I would like to have an extra flag on jj rebase and jj new, which would call the equivalent of jj simplify-parents -r 'reachable(C, mutable())' where C is the commit(s) being rebased or create.

Describe alternatives you've considered
I've been using two commands in succession right now.

Additional context
To give an example, here is my current development state for Clippy, I think it easy to understand why simplifying the tree is important for me:

[bnjmnt4n]

Related FR: #4708