rebase: allow -r to accept multiple revisions

[bnjmnt4n]

Closes #1158.

Checklist

If applicable:

• 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)
• I have added tests to cover my changes

[martinvonz]

How much will this conflict with #3396 (--insert-before/-after)? I'd rather see this PR merged first, both because I think it's more useful, and because it seems important to keep it in mind when designing --insert-before/-after. Do you agree?

EDIT: "because it seems important to keep it in mind when designing" can of course apply in the other direction too, but I think it seems like a bigger risk that --insert-before/-after with single-commit -r would be implemented in a way that's hard to adapt to multi-commit -r than the other way around

[bnjmnt4n]

Agreed, I would rather this PR get merged first. It’s functionally ready, but I probably need to do some minor code cleanup as well.

[martinvonz]

Thank you!