cli: make jj split more explicit about filesets

[fowles]

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

[PhilipMetzger]

While I really like how simple this looks, we've only had inconclusive discussions around multi-splitting in Discord. This also resurfaces #3879/#3549 (comment) which hasn't gotten any valuable traction.

[PhilipMetzger]

LG

We need to know how much the feature is actually needed as by Martin.

[arxanas]

This seems like a great change to me. I was also personally unaware that jj split, etc. accepted filesets rather than just paths. We should update all the other places that accept filesets at some point as well. (I'll leave @martinvonz to give the final sign-off since this is set to auto-merge.)

In principle: I suppose a user might check the help text and be a little put off or scared by talk of "filesets" instead of just "files" or "paths". But I'm optimistic they'll have seen enough example usage to just ignore what they don't understand and pass paths normally, and treat "filesets" as a black box abstraction if necessary.

[martinvonz]

I think every command that uses parse_file_patterns() could use the same update. I can try to follow up with that.

[yuja]

I think every command that uses parse_file_patterns() could use the same update. I can try to follow up with that.

Maybe we should update -r <REVISION> to -r <REVSET> for consistency? (I don't think we'll need to rename internal variables.)