🤖 fix: fetch when remote ref moves (#947)

## Summary
- fetch when remote ref changes even if object already present locally
- keep fetch skip when tracking ref matches remote
- add regression test for remote ref move with existing object

## Testing
- make static-check
- bun test src/common/utils/git/gitStatus.fetch.test.ts

_Generated with `mux`_.

Author: ammar-agent

src/common/utils/git/gitStatus.fetch.test.ts

@@ -0,0 +1,62 @@
+import { mkdtemp, rm, writeFile } from "fs/promises";
+import * as path from "path";
+import * as os from "os";
+import { execSync } from "child_process";
+
+import { GIT_FETCH_SCRIPT } from "./gitStatus";
+
+describe("GIT_FETCH_SCRIPT", () => {
+  test("fetches when remote ref moves to a commit already present locally", async () => {
+    const tempDir = await mkdtemp(path.join(os.tmpdir(), "mux-git-fetch-"));
+    const originDir = path.join(tempDir, "origin.git");
+    const workspaceDir = path.join(tempDir, "workspace");
+
+    const run = (cmd: string, cwd?: string) =>
+      execSync(cmd, { cwd, stdio: "pipe" }).toString().trim();
+
+    try {
+      // Initialize bare remote and clone it
+      run(`git init --bare ${originDir}`);
+      run(`git clone ${originDir} ${workspaceDir}`);
+
+      // Basic git identity configuration
+      run('git config user.email "test@example.com"', workspaceDir);
+      run('git config user.name "Test User"', workspaceDir);
+      run("git config commit.gpgsign false", workspaceDir);
+
+      // Seed main with an initial commit
+      await writeFile(path.join(workspaceDir, "README.md"), "init\n");
+      run("git add README.md", workspaceDir);
+      run('git commit -m "init"', workspaceDir);
+      run("git branch -M main", workspaceDir);
+      run("git push -u origin main", workspaceDir);
+
+      // Ensure remote HEAD points to main for deterministic primary branch detection
+      run("git symbolic-ref HEAD refs/heads/main", originDir);
+
+      // Create a commit on a feature branch (object exists locally)
+      run("git checkout -b feature", workspaceDir);
+      await writeFile(path.join(workspaceDir, "feature.txt"), "feature\n");
+      run("git add feature.txt", workspaceDir);
+      run('git commit -m "feature"', workspaceDir);
+      const featureSha = run("git rev-parse feature", workspaceDir);
+
+      // Push the feature branch so the remote has the object but main stays old
+      run("git push origin feature", workspaceDir);
+
+      // Move remote main to the feature commit without updating local tracking ref
+      run(`git update-ref refs/heads/main ${featureSha}`, originDir);
+
+      const localBefore = run("git rev-parse origin/main", workspaceDir);
+      expect(localBefore).not.toBe(featureSha);
+
+      // Run the optimized fetch script (should update origin/main)
+      run(GIT_FETCH_SCRIPT, workspaceDir);
+
+      const localAfter = run("git rev-parse origin/main", workspaceDir);
+      expect(localAfter).toBe(featureSha);
+    } finally {
+      await rm(tempDir, { recursive: true, force: true });
+    }
+  }, 20000);
+});

src/common/utils/git/gitStatus.ts

@@ -117,13 +117,16 @@ if [ -z "$REMOTE_SHA" ]; then
   exit 0
 fi
 
-# Check if SHA exists locally (no lock)
-if git cat-file -e "$REMOTE_SHA" 2>/dev/null; then
-  echo "SKIP: Remote SHA already local"
+# Check current local remote-tracking ref (no lock)
+LOCAL_SHA=$(git rev-parse --verify "refs/remotes/origin/$PRIMARY_BRANCH" 2>/dev/null || echo "")
+
+# If local tracking ref already matches remote, skip fetch
+if [ "$LOCAL_SHA" = "$REMOTE_SHA" ]; then
+  echo "SKIP: Remote SHA already fetched"
   exit 0
 fi
 
-# Remote has new commits we don't have - fetch them
+# Remote has new commits or ref moved - fetch updates
 git -c protocol.version=2 \\
     -c fetch.negotiationAlgorithm=skipping \\
     fetch origin \\