Use merge base when calculating changed files (#16709)

`get-pr-info.outputs.base.sha` does not actually give the merge base, but merely the tip of the target branch. Calculate the merge base and pass it to the `changed-files` step.

Authors:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

Approvers:
  - Ray Douglass (https://github.com/raydouglass)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #16709

.github/workflows/pr.yaml

@@ -56,14 +56,21 @@ jobs:
       - name: Checkout code repo
         uses: actions/checkout@v4
         with:
-          ref: ${{ inputs.sha }}
-          fetch-depth: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).commits }}
+          fetch-depth: 0
           persist-credentials: false
+      - name: Calculate merge base
+        id: calculate-merge-base
+        env:
+          PR_SHA: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).head.sha }}
+          BASE_SHA: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).base.sha }}
+        run: |
+          (echo -n "merge-base="; git merge-base "$BASE_SHA" "$PR_SHA") > "$GITHUB_OUTPUT"
       - name: Get changed files
         id: changed-files
         uses: tj-actions/changed-files@v45
         with:
-          base_sha: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).base.sha }}
+          base_sha: ${{ steps.calculate-merge-base.outputs.merge-base }}
+          sha: ${{ fromJSON(steps.get-pr-info.outputs.pr-info).head.sha }}
           files_yaml: |
             cpp:
               - '**'