Add shallow fetch to improve performance for large repositories (anthropics#53)

* Add shallow fetch to improve performance for large repositories

This change adds `--depth=1` to git fetch operations to perform shallow
fetches instead of full history downloads. This significantly reduces
checkout time for large repositories as reported in issue anthropics#52.

Changes:
- Line 55: Added --depth=1 to PR branch fetch
- Line 102: Added --depth=1 to new branch fetch

Fixes anthropics#52

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* fetch 50 commits for PRs

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

Author: ashwin-ant

src/github/operations/branch.ts

@@ -51,8 +51,9 @@ export async function setupBranch(
 
       const branchName = prData.headRefName;
 
-      // Execute git commands to checkout PR branch
-      await $`git fetch origin ${branchName}`;
+      // Execute git commands to checkout PR branch (shallow fetch for performance)
+      // Fetch the branch with a depth of 20 to avoid fetching too much history, while still allowing for some context
+      await $`git fetch origin --depth=20 ${branchName}`;
       await $`git checkout ${branchName}`;
 
       console.log(`Successfully checked out PR branch for PR #${entityNumber}`);
@@ -98,8 +99,8 @@ export async function setupBranch(
       sha: currentSHA,
     });
 
-    // Checkout the new branch
-    await $`git fetch origin ${newBranch}`;
+    // Checkout the new branch (shallow fetch for performance)
+    await $`git fetch origin --depth=1 ${newBranch}`;
     await $`git checkout ${newBranch}`;
 
     console.log(