Add a git: copy branch name action (#54702)

I frequently use the branch name copy action in GitHub Desktop.
I want to do this in Zed.

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [X] Unsafe blocks (if any) have justifying comments
- [X] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Release Notes:

- Added a `git: copy branch name` action.

crates/git/src/git.rs

@@ -105,6 +105,8 @@ actions!(
         ViewCommit,
         /// Adds a file to .gitignore.
         AddToGitignore,
+        /// Copies the current branch name to the clipboard.
+        CopyBranchName,
     ]
 );
 

crates/git_ui/src/git_ui.rs

@@ -17,8 +17,8 @@ use git::{
     status::{FileStatus, StatusCode, UnmergedStatus, UnmergedStatusCode},
 };
 use gpui::{
-    App, Context, DismissEvent, Entity, EventEmitter, FocusHandle, Focusable, SharedString,
-    Subscription, Task, Window,
+    App, ClipboardItem, Context, DismissEvent, Entity, EventEmitter, FocusHandle, Focusable,
+    SharedString, Subscription, Task, Window,
 };
 use menu::{Cancel, Confirm};
 use project::git_store::Repository;
@@ -270,6 +270,9 @@ pub fn init(cx: &mut App) {
         workspace.register_action(|workspace, _: &git::RenameBranch, window, cx| {
             rename_current_branch(workspace, window, cx);
         });
+        workspace.register_action(|workspace, _: &git::CopyBranchName, _, cx| {
+            copy_branch_name(workspace, cx);
+        });
         workspace.register_action(show_ref_picker);
         workspace.register_action(
             |workspace, action: &DiffClipboardWithSelectionData, window, cx| {
@@ -465,6 +468,20 @@ fn rename_current_branch(
     });
 }
 
+fn copy_branch_name(workspace: &mut Workspace, cx: &mut Context<Workspace>) {
+    let Some(panel) = workspace.panel::<GitPanel>(cx) else {
+        return;
+    };
+    let branch_name = panel.update(cx, |panel, cx| {
+        let repo = panel.active_repository.as_ref()?;
+        let repo = repo.read(cx);
+        repo.branch.as_ref().map(|branch| branch.name().to_string())
+    });
+    if let Some(name) = branch_name {
+        cx.write_to_clipboard(ClipboardItem::new_string(name));
+    }
+}
+
 struct RefPickerModal {
     editor: Entity<Editor>,
     repo: Entity<Repository>,