feat: vcs: support Jujutsu as a diff-provider

[poliorcetics]

Built on top of #9951, I'll be waiting for it to be merged to un-draft this

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Jujutsu (jj) is a new change-based VCS (whereas git is branch-based).

In this PR, I add the ability for helix to get the diffs and current head, behind a feature called jj that is active by default. That would make it the first editor I know off that officially handles that VCS!

To handle all current, future and private backends (Google already has one I believe), I instead made it so Helix can behave as a diff tool for Jujutsu and then use that as a subcommand to get the diff base.

For the head, I simply used the templating system to extract all relevant informations.

Jujutsu has a library, called jj-lib but it's not ready for use in third party programs and wouldn't fix the issue of custom backends anyway.

Testing

Since testing needs jj installed, I haven't written any for Helix yet to discuss how to do it:

1. Ignore the issue. I don't like it much, but it doesn't cost CI time and doesn't need any change. For a feature that probably won't be used much yet, that's probably ok for a time
2. Install jj in CI and use it to test the new feature