cli: don't allow multiple values for jj rebase and jj untrack

Somehow I had thought that setting `clap(required = true)` on a
list-type argument was not enough to convince clap to require at least
one occurrence, but it seems that it does. The `min_values = 1` I had
added to get it to work actually means that we accept multiple values
passed to a single argument (e.g. `jj rebase -d x y z`), which is not
what I had intended.

src/commands.rs

@@ -1042,7 +1042,7 @@ struct CheckoutArgs {
 #[derive(clap::Args, Clone, Debug)]
 struct UntrackArgs {
     /// Paths to untrack
-    #[clap(required = true, min_values = 1)]
+    #[clap(required = true)]
     paths: Vec<String>,
 }
 
@@ -1422,7 +1422,7 @@ struct RebaseArgs {
     #[clap(long, short)]
     source: Option<String>,
     /// The revision to rebase onto
-    #[clap(long, short, required = true, min_values = 1)]
+    #[clap(long, short, required = true)]
     destination: Vec<String>,
 }