cli new: Allow --before/--after without a value to default to @

[ilyagr]

Without this, I find it a bit jarring that jj new works but jj new --before does not. By contrast, since jj rebase does not currently work, I don't think jj rebase --before should either.

Note that jj new --before @ another_revision is invalid, so jj new --before another_revision can only be parsed correctly in one way. I am slightly concerned that clap might forbids this in the future even in the cases where a human can tell there is no ambiguity, but I'm hoping for the best.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• n/a I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[martinvonz]

Without this, I find it a bit jarring that jj new works but jj new --before does not.

I would not expect jj new --before to work. Maybe it's just because I'm used to Mercurial's argument parsing. Either way, I didn't think we have and existing flags with optional values, so I think we should think carefully before we add one. I assume it will only be possible to do it for commands that don't take any positional arguments, which limits the commands where we can use it even if we wanted to.

[ilyagr]

By the way, jj new --before used to work before we added the ability to use both --before and --after in the same command, since it was a boolean flag back then. I used that often, and I still haven't gotten used to it going away. My sense of "jj new assumes @" is strong. I don't know whether I was the only one.

I do not propose we use these very often, but jj new seems pretty safe to me as I explained above and as the tests show. Still, I can understand it if people are worried, and perhaps I will get used to it eventually.