Add branch rename action to Git panel (#38273)

Reopening #35136, cc @launay12u

Release Notes:

- git: added `git: rename branch` action to rename a branch (`git branch
-m`)

---------

Co-authored-by: Guillaume Launay <guillaume.launay@paylead.fr>
Co-authored-by: Peter Tripp <petertripp@gmail.com>

crates/fs/src/fake_git_repo.rs

@@ -1,5 +1,5 @@
 use crate::{FakeFs, FakeFsEntry, Fs};
-use anyhow::{Context as _, Result};
+use anyhow::{Context as _, Result, bail};
 use collections::{HashMap, HashSet};
 use futures::future::{self, BoxFuture, join_all};
 use git::{
@@ -354,6 +354,19 @@ impl GitRepository for FakeGitRepository {
         })
     }
 
+    fn rename_branch(&self, branch: String, new_name: String) -> BoxFuture<'_, Result<()>> {
+        self.with_state_async(true, move |state| {
+            if !state.branches.remove(&branch) {
+                bail!("no such branch: {branch}");
+            }
+            state.branches.insert(new_name.clone());
+            if state.current_branch_name == Some(branch) {
+                state.current_branch_name = Some(new_name);
+            }
+            Ok(())
+        })
+    }
+
     fn blame(&self, path: RepoPath, _content: Rope) -> BoxFuture<'_, Result<git::blame::Blame>> {
         self.with_state_async(false, move |state| {
             state