cli: Ignore `immutable()` in `jj git push` conflict/desc/user checks

[mlcui-corp]

Fixes #3029.

Checklist

If applicable:

• [X] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [X] I have added tests to cover my changes

[mlcui-corp]

PTAL, especially with the new changes in cli/src/revset_util.rs.