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 +124,22 @@ pub struct BranchForgetArgs {
     pub glob: Vec<StringPattern>,
 }
 
+/// Rename an existing branch pointed to by a specified commit.
+#[derive(clap::Args, Clone, Debug)]
+pub struct BranchRenameArgs {
+    /// The old name of the branch.
+    #[arg(required = true)]
+    pub old: String,
+
+    /// The old branch's target revision.
+    #[arg(long, short)]
+    pub revision: Option<RevisionArg>,
+
+    /// The new name of the branch.
+    #[arg(required = true)]
+    pub new: String,
+}
+
 /// Update an existing branch to point to a certain commit.
 #[derive(clap::Args, Clone, Debug)]
 pub struct BranchSetArgs {
@@ -248,6 +266,7 @@ 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 +320,42 @@ 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 target_commit =
+        workspace_command.resolve_single_rev(args.revision.as_deref().unwrap_or("@"), ui)?;
+    let repo = workspace_command.repo().as_ref();
+    let new_branch = &args.new;
+    let new_target = repo.view().get_local_branch(new_branch);
+    if !new_target.is_absent() {
+        return Err(user_error_with_hint(
+            format!("Branch name conflict: {new_branch}"),
+            "Target branch name already exists.",
+        ));
+    }
+
+    let old_branch = &args.old;
+    let mut tx = workspace_command.start_transaction();
+    tx.mut_repo()
+        .set_local_branch_target(new_branch, RefTarget::normal(target_commit.id().clone()));
+    tx.mut_repo()
+        .set_local_branch_target(old_branch, RefTarget::absent());
+    tx.finish(
+        ui,
+        format!(
+            "rename {} to {}; still point to commit {}",
+            make_branch_term(&[old_branch]),
+            make_branch_term(&[new_branch]),
+            target_commit.id().hex()
+        ),
+    )?;
+    Ok(())
+}
+
 fn cmd_branch_set(
     ui: &mut Ui,
     command: &CommandHelper,