cli: disallow to create new branch by "jj branch set"

Per discussion in https://github.com/martinvonz/jj/discussions/2555. I'm
okay with either way, but it's confusing if we had "branch create" and
"branch set" and both of these could create a new branch.

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * The `remote_branches()` revset no longer includes branches exported to the Git
   repository (so called Git-tracking branches.)
 
+* `jj branch set` no longer creates a new branch. Use `jj branch create`
+  instead.
+
 ### New features
 
 * `jj workspace add` can now take _multiple_ `--revision` arguments, which will

cli/src/commands/branch.rs

@@ -123,7 +123,7 @@ pub struct BranchForgetArgs {
     pub glob: Vec<StringPattern>,
 }
 
-/// Update a given branch to point to a certain commit.
+/// Update an existing branch to point to a certain commit.
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchSetArgs {
     /// The branch's target revision.
@@ -298,6 +298,15 @@ fn cmd_branch_set(
         workspace_command.resolve_single_rev(args.revision.as_deref().unwrap_or("@"), ui)?;
     let repo = workspace_command.repo().as_ref();
     let branch_names = &args.names;
+    for name in branch_names {
+        let old_target = repo.view().get_local_branch(name);
+        if old_target.is_absent() {
+            return Err(user_error_with_hint(
+                format!("No such branch: {name}"),
+                "Use `jj branch create` to create it.",
+            ));
+        }
+    }
     if !args.allow_backwards
         && !branch_names
             .iter()

cli/tests/test_branch_command.rs

@@ -98,6 +98,12 @@ fn test_branch_move() {
     test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
     let repo_path = test_env.env_root().join("repo");
 
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["branch", "set", "foo"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: No such branch: foo
+    Hint: Use `jj branch create` to create it.
+    "###);
+
     let (_stdout, stderr) = test_env.jj_cmd_ok(&repo_path, &["branch", "create", "foo"]);
     insta::assert_snapshot!(stderr, @"");