cli: branch: reject empty branch name consistently by "set"

Though "branch set" can't create new branch, this should provide a better error
message.

cli/src/commands/branch/create.rs

@@ -29,7 +29,7 @@ pub struct BranchCreateArgs {
     revision: Option<RevisionArg>,
 
     /// The branches to create
-    #[arg(required = true, value_parser=NonEmptyStringValueParser::new())]
+    #[arg(required = true, value_parser = NonEmptyStringValueParser::new())]
     names: Vec<String>,
 }
 

cli/src/commands/branch/set.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use clap::builder::NonEmptyStringValueParser;
 use jj_lib::object_id::ObjectId as _;
 use jj_lib::op_store::RefTarget;
 
@@ -32,7 +33,7 @@ pub struct BranchSetArgs {
     allow_backwards: bool,
 
     /// The branches to update
-    #[arg(required = true)]
+    #[arg(required = true, value_parser = NonEmptyStringValueParser::new())]
     names: Vec<String>,
 }
 

cli/tests/test_branch_command.rs

@@ -88,6 +88,13 @@ fn test_branch_empty_name() {
 
     For more information, try '--help'.
     "###);
+
+    let stderr = test_env.jj_cmd_cli_error(&repo_path, &["branch", "set", ""]);
+    insta::assert_snapshot!(stderr, @r###"
+    error: a value is required for '<NAMES>...' but none was supplied
+
+    For more information, try '--help'.
+    "###);
 }
 
 #[test]