Implement a rename subcommand for the branch command.

This is really a simple change that does the following in a transaction:
* Set the new branch name to point to the same commit as the old branch name.
* Set the old branch name to point to no commit (hence deleting the old name).

Before it starts, it confirms that the new branch name is not already in use.

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### New features
 
 * Information about new and resolved conflicts is now printed by every command.
+* `jj branch` has gained a new `rename` subcommand that allows changing a branch
+  name atomically. `jj branch help rename` for details.
 
 ### Fixed bugs
 

cli/src/commands/branch.rs

@@ -34,6 +34,8 @@ pub enum BranchSubcommand {
     Forget(BranchForgetArgs),
     #[command(visible_alias("l"))]
     List(BranchListArgs),
+    #[command(visible_alias("r"))]
+    Rename(BranchRenameArgs),
     #[command(visible_alias("s"))]
     Set(BranchSetArgs),
     Track(BranchTrackArgs),
@@ -122,6 +124,19 @@ pub struct BranchForgetArgs {
     pub glob: Vec<StringPattern>,
 }
 
+/// Rename `old` branch name to `new` branch name.
+///
+/// The new branch name points at the same commit as the old
+/// branch name.
+#[derive(clap::Args, Clone, Debug)]
+pub struct BranchRenameArgs {
+    /// The old name of the branch.
+    pub old: String,
+
+    /// The new name of the branch.
+    pub new: String,
+}
+
 /// Update an existing branch to point to a certain commit.
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchSetArgs {
@@ -248,6 +263,7 @@ pub fn cmd_branch(
 ) -> Result<(), CommandError> {
     match subcommand {
         BranchSubcommand::Create(sub_args) => cmd_branch_create(ui, command, sub_args),
+        BranchSubcommand::Rename(sub_args) => cmd_branch_rename(ui, command, sub_args),
         BranchSubcommand::Set(sub_args) => cmd_branch_set(ui, command, sub_args),
         BranchSubcommand::Delete(sub_args) => cmd_branch_delete(ui, command, sub_args),
         BranchSubcommand::Forget(sub_args) => cmd_branch_forget(ui, command, sub_args),
@@ -301,6 +317,41 @@ fn cmd_branch_create(
     Ok(())
 }
 
+fn cmd_branch_rename(
+    ui: &mut Ui,
+    command: &CommandHelper,
+    args: &BranchRenameArgs,
+) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let repo = workspace_command.repo().as_ref();
+    let old_branch = &args.old;
+    // call clone because .set_local_branch_target takes ownership
+    // of the target_commit, so we need to create a copy that is no
+    // longer owned by the workspace.
+    let target_commit = repo.view().get_local_branch(old_branch).clone();
+    if target_commit.is_absent() {
+        return Err(user_error_with_hint(format!(
+            "No such branch: {old_branch}"
+        )));
+    }
+
+    let new_branch = &args.new;
+    let mut tx = workspace_command.start_transaction();
+    tx.mut_repo()
+        .set_local_branch_target(new_branch, target_commit);
+    tx.mut_repo()
+        .set_local_branch_target(old_branch, RefTarget::absent());
+    tx.finish(
+        ui,
+        format!(
+            "rename {} to {}",
+            make_branch_term(&[old_branch]),
+            make_branch_term(&[new_branch]),
+        ),
+    )?;
+    Ok(())
+}
+
 fn cmd_branch_set(
     ui: &mut Ui,
     command: &CommandHelper,