Revert "chore(diff-flat): remove branch workarounds"

This reverts commit 6cc19fdfe2b2ec52183bdc7ad3f87e872e9a1d93.

.github/workflows/pr-review-companion.yml

diff --git a/.github/workflows/pr-review-companion.yml b/.github/workflows/pr-review-companion.yml
index 27dc15b..d9364b4 100644
--- a/.github/workflows/pr-review-companion.yml
+++ b/.github/workflows/pr-review-companion.yml
@@ -21,6 +21,8 @@
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          ref: flat-diff
 
       - name: Setup Node.js
         uses: actions/setup-node@v4

scripts/diff-flat.ts

diff --git a/scripts/diff-flat.ts b/scripts/diff-flat.ts
index 3185831..8d32f8c 100644
--- a/scripts/diff-flat.ts
+++ b/scripts/diff-flat.ts
@@ -644,6 +644,14 @@
     options.base = 'origin/main';
   }
 
+  if (
+    options.head === 'HEAD' &&
+    spawn('git', ['branch', '--show-current']) === 'flat-diff'
+  ) {
+    // Workaround: Compare first positional parameter against origin/main.
+    [options.base, options.head] = [options.head, 'origin/main'];
+  }
+
   const remote =
     spawn('git', ['remote', '-v'])
       .split('\n')