cli: require at least one destination for rebase

Rebasing onto no commits (i.e. onto the root commit) is almost
definitely not what you meant to do if you run `jj rebase` (without
any arguments).

CHANGELOG.md

@@ -37,6 +37,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj untrack` now requires at least one path (allowing no arguments was a UX
   bug).
 
+* `jj rebase` now requires at least one destination (allowing no arguments was a
+  UX bug).
+
 * You now get a proper error message instead of a crash when `$EDITOR` doesn't
   exist or exits with an error.
 

src/commands.rs

@@ -1422,7 +1422,7 @@ struct RebaseArgs {
     #[clap(long, short)]
     source: Option<String>,
     /// The revision to rebase onto
-    #[clap(long, short)]
+    #[clap(long, short, required = true, min_values = 1)]
     destination: Vec<String>,
 }