cli: add test to ensure the default config has no symbol aliases

[thoughtpolice]

In #4432 I submitted a patch to ship a symbol alias in the default configuration. However, Yuya quickly pointed out that this isn't acceptable because symbols override all branch and tag names, meaning that the name at would have at best been taken away from users, and at worst been very confusing for them down the road had they ever tried to use it.

I should have thought of this myself (I encountered it long ago when first using Jujutsu, actually) but didn't. Such is life.

But this didn't cause any tests to fail, as it was not immediately clear this would impede anything or cause downstream behavioral changes; the problems it adds are actually latent and it's very possible someone will want to make the same mistake in the future. And Yuya might not be able to stop them (me) in time; requiring him to go out of his way and then handle the fallout. I'm not a fan of this.

Instead, let's patch this latent hole for good, or until we decide otherwise, by applying the Beyonce Rule: "if you liked that behavior, then you shoulda put a test on it."

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

[thoughtpolice]

@yuja The "is this a symbol" check is a complete hack, but maybe that's OK. Let me know what you think.

[yuja]

@yuja The "is this a symbol" check is a complete hack, but maybe that's OK. Let me know what you think.

That's good. We could also use RevsetAliasesMap::insert() and function_names().

That said, I think a comment like # Don't add default symbol aliases because ... in config/revsets.toml is good enough.