This allows us to use "if(description,)" to test empty description. And
I think this change is unavoidable if we want to add support for commit
template.
The `indexmap` crate is used to make `duplicate`'s output have a sane order,
making it easier to test.
It's also used later to remove duplicate revisions in the `abandon` command.
Fixes https://github.com/martinvonz/jj/issues/1050
Thanks to Martin for suggesting the exact fix.
The tests go into the new tests/test_duplicate_command.rs, which will be
expanded shortly with other tests depending on this bugfix.
