FR: Warn when revision is used without -r for diff and split

[tp-woven]

Since diff and split default to -r @, it is easy to forget to add -r when using them for another revision. When this happens, jj treats the argument as a file name that usually doesn't match anything and proceeds to report and empty diff.

Instead, jj should show a warning when the pattern doesn't match any actual files, similar to log. Potentially, we could also (or instead) check whether the argument actually matches a revision first.

Related Discord discussion: https://discord.com/channels/968932220549103686/968932220549103689/1098397923819135036

[ilyagr]

I think the related #505 was the first issue I filed here. Since then, I started to think that the same approach as with jj log might be easier and equally effective.

[tp-woven]

Thanks, I'll close this as a duplicate of #505 then.

[ilyagr]

OK, I'll change the name of that bug then to make it more general.