Fix markdown-fail-fast Get Changes step script (#7045)

This has been failing silently:
https://github.com/open-telemetry/opentelemetry-go/actions/runs/16361094932/job/46229000882

- Fix the variable handling
- Add debugging `echo`s
- Reduce permissions to minimum needed

.github/workflows/markdown-fail-fast.yml

@@ -4,8 +4,8 @@ on:
   push:
   pull_request:
 
-# Declare default permissions as read only.
-permissions: read-all
+permissions:
+  contents: read
 
 jobs:
   changedfiles:
@@ -20,8 +20,24 @@ jobs:
           fetch-depth: 0
       - name: Get changed files
         id: changes
+        env:
+          PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+        shell: bash
         run: |
-          echo "md=$(git diff --name-only --diff-filter=ACMRTUXB origin/${{ github.event.pull_request.base.ref }} ${{ github.event.pull_request.head.sha }} | grep .md$ | xargs)" >> $GITHUB_OUTPUT
+          echo "PR_HEAD_SHA=$PR_HEAD_SHA"
+
+          base="$(git merge-base origin/main $PR_HEAD_SHA)"
+          echo "BASE_REF=$base"
+
+          changed="$(git diff --name-only --diff-filter=ACMRTUXB $base $PR_HEAD_SHA)"
+          echo "CHANGED=$changed"
+
+          filtered="$(grep '.md$' <<<"$changed" || true)"
+          echo "filtered=$filtered"
+
+          md="$(echo $filtered | xargs -d '\n' echo)"
+          echo "md=$md"
+          echo "md=$md" >> $GITHUB_OUTPUT
 
   lint:
     name: lint markdown files