feat: implement branch reuse logic and add tests for existing branches

Author: CasperKristiansson

docs/tasks.md

@@ -233,7 +233,7 @@ Use Context7 MCP for up to date documentation.
         Validate `track` as `/^\d+\.\d+$/`.
         Verify: Bad inputs fail fast.
 
-38. [ ] **Concurrency control**
+38. [x] **Concurrency control**
         Check existing ref before branch create. Document workflow `concurrency`.
         Verify: Parallel runs yield one PR.
 

src/git/branch.ts

@@ -20,6 +20,15 @@ export interface BranchCommitResult {
   filesCommitted: string[];
 }
 
+async function branchExists(branch: string, repoPath: string): Promise<boolean> {
+  try {
+    await runGit(['show-ref', '--verify', `refs/heads/${branch}`], repoPath);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 async function runGit(
   args: string[],
   repoPath: string,
@@ -67,7 +76,11 @@ export async function createBranchAndCommit(
 
   const branch = `${branchPrefix}${track}`;
 
-  await runGit(['checkout', '-B', branch], repoPath);
+  if (await branchExists(branch, repoPath)) {
+    await runGit(['checkout', branch], repoPath);
+  } else {
+    await runGit(['checkout', '-B', branch], repoPath);
+  }
   await stageFiles(files, repoPath);
   const stagedFiles = await getStagedFiles(repoPath);
 

tests/git-branch.test.ts

@@ -74,4 +74,31 @@ describe('createBranchAndCommit', () => {
     const status = await runGit(['status', '--short'], repoDir);
     expect(status).toBe('');
   });
+
+  it('reuses existing branch when present', async () => {
+    const filePath = path.join(repoDir, 'Dockerfile');
+    await writeFile(filePath, 'FROM python:3.11.8-slim\n');
+
+    // First run creates the branch and commit.
+    await createBranchAndCommit({
+      repoPath: repoDir,
+      track: '3.11',
+      files: ['Dockerfile'],
+      commitMessage: 'chore: bump python to 3.11.9',
+    });
+
+    // Modify file again and ensure second run keeps branch without recreation errors.
+    await writeFile(filePath, 'FROM python:3.11.9-slim\n');
+
+    const result = await createBranchAndCommit({
+      repoPath: repoDir,
+      track: '3.11',
+      files: ['Dockerfile'],
+      commitMessage: 'chore: bump python to 3.11.10',
+    });
+
+    expect(result.branch).toBe('chore/bump-python-3.11');
+    const commitCount = await runGit(['rev-list', '--count', 'chore/bump-python-3.11'], repoDir);
+    expect(Number.parseInt(commitCount, 10)).toBeGreaterThanOrEqual(2);
+  });
 });