FR: Recover better from `jj split <change ID>`

[zygoloid]

Is your feature request related to a problem? Please describe.
I frequently forget the -r when running jj split. If I run jj split abc, because abc doesn't match any filenames, jj split decides that I want to split out an empty commit, and drops me straight into an editor to enter a commit message for the new empty commit. Depending on my editor, I may have no good way out of this other than to quit twice then jj undo.

jj split produces a warning, but I don't get to see it, because my editor has taken over the terminal. And if my editor doesn't use altscreen, then I don't even get to see it when I quit the editor!

Describe the solution you'd like
Really, anything better would be nice.

Describe alternatives you've considered
jj split abc, if abc matches no paths (or possibly if it matches no changes), could fail unless I specify --allow-empty or similar, at least in the case where abc is a valid change id. (See also #9245, which is a variant of this.)

Or jj could reject filesets that match no files in general. (I guess with a global flag to override.)

Or jj could detect when a filename input is a valid change id in general, and ask that you use a ./ prefix if that's what you really meant. (Probably not great for use from scripts.)

Or the editor that opens to enter the commit message could explain what happened, and maybe some special commit message such as "undo" or "abort" would bail out instead of creating the empty commit. I even think it'd be worth pre-filling the commit message with the special "abort" message so that quitting the editor without changing the message would abandon the split.

[PhilipMetzger]

Or the editor that opens to enter the commit message could explain what happened, and maybe some special commit message such as "undo" or "abort" would bail out instead of creating the empty commit.

This sounds like #4414 (comment).

The current bug description also sounds like a dup of #4414.

[zygoloid]

The current bug description also sounds like a dup of #4414.

I don't think that it is a duplicate. This bug is about an overall user experience issue, not about a particular solution to it or a particular aspect of how it manifests. And 4414 alone wouldn't solve this unless the commit message is also updated to explain what happened, for the reason described in the second paragraph.

[josephlou5]

jj split decides that I want to split out an empty commit

This is kind of strange to me 🙃 Looks like the effect is to create a new child change with all files (because the user "selected nothing" to keep in the original change). I feel like the edge case of selecting nothing with a fileset (as opposed to opening your diff editor and then selecting nothing) indeed seems like it deserves special handling. Personally I can't come up with a situation in which I would want this behavior to happen (then again I don't use jj split), but a flag to force the split anyway seems fine.

[yuja]

Dup of #1155 and #4084 ?