Wrong warnings when finishing `split` with an empty side

[Vampire]

Description

Disclaimer:
If #9245 gets done, this should become obsolete.
But if that issue gets rejected or needs some time to be done,
this could be fixed instead or in the meantime.

If you select no changes in a split, you get an empty original commit
and a new child commit with all changes.
But the warning after the command says

Warning: No changes have been selected, so the new revision will be empty

which is wrong.

Likewise, if you select all changes, you get an empty new commit and original commit stays untouched besides the needless commit date change.
But the warning says

Warning: All changes have been selected, so the original revision will become empty

And finally, if you split an empty revision (which should not even be possible according to docs),
you get the two warnings

Warning: Empty diff - won't run diff editor.
Warning: All changes have been selected, so the original revision will become empty

The second one is not wrong, but it is a bit useless.
And it also is only correct now because "All" and "No" changes are the same in this case.

Steps to Reproduce the Problem

1. jj split
2. select all or no changes

Expected Behavior

correct message

Actual Behavior

wrong message

Specifications

• Platform: all
• Version: 0.40.0

[martinvonz]

I think it's correct when you use -o/-A/-B. It would also be correct in the default case (parent/child case) if #6466 gets merged. (But I think you're right that they're currently incorrect in the parent/child case.)

[Vampire]

Oh, that's even more confusing.
That when using -o/-A/-B suddenly the logic changes where the selected changes go.
In the normal case what you select keeps the changes.
So yeah, #6466 would (should) probably "fix" the warnings here and remove that confusion.