Diff against the merge base instead of the BASE_REF tip

The previous shallow fetch of BASE_REF and `git diff BASE_REF..HEAD`
produced a meaningless cross-tree diff when the PR head was rebased
onto an older base commit: everything BASE_REF gained since the rebase
point showed up as "removed in PR". Resolve the merge base via the
GitHub compare API and fetch that commit by SHA so the diff matches
GitHub's "Files changed" semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Seldaek

action.yml

@@ -93,20 +93,35 @@ runs:
         fetch-depth: 1
         persist-credentials: false
 
-    - name: Fetch base ref
+    - name: Fetch merge base
+      id: base
       if: steps.pr.outputs.number
       shell: bash
       env:
+        GH_TOKEN: ${{ github.token }}
+        REPO: ${{ github.repository }}
         BASE_REF: ${{ steps.pr.outputs.base-ref }}
-      run: git fetch --no-tags --depth=1 origin "${BASE_REF}:refs/remotes/origin/${BASE_REF}"
+      run: |
+        # Resolve the merge base server-side: shallow fetches mean the local
+        # repo can't compute it, and diffing against the BASE_REF tip produces
+        # garbage when the PR is rebased onto an older commit (everything main
+        # has gained since the rebase point shows up as "removed in PR").
+        head_sha=$(git rev-parse HEAD)
+        merge_base=$(gh api "repos/${REPO}/compare/${BASE_REF}...${head_sha}" --jq '.merge_base_commit.sha')
+        if [[ -z "${merge_base}" || "${merge_base}" == "null" ]]; then
+            echo "::error::Could not determine merge base for ${BASE_REF}...${head_sha}"
+            exit 1
+        fi
+        git fetch --no-tags --depth=1 origin "${merge_base}"
+        echo "merge-base=${merge_base}" >> "$GITHUB_OUTPUT"
 
     - name: Preflight scan
       id: preflight
       if: steps.pr.outputs.number
       shell: bash
       working-directory: ${{ inputs.working-directory }}
       env:
-        BASE_REF: origin/${{ steps.pr.outputs.base-ref }}
+        BASE_REF: ${{ steps.base.outputs.merge-base }}
         PATHS_GLOB: ${{ inputs.paths }}
         OUTPUT_FILE: api-surface-preflight.txt
         ACTION_PATH: ${{ github.action_path }}
@@ -155,7 +170,7 @@ runs:
       shell: bash
       working-directory: ${{ inputs.working-directory }}
       env:
-        BASE_REF: origin/${{ steps.pr.outputs.base-ref }}
+        BASE_REF: ${{ steps.base.outputs.merge-base }}
         PATHS_GLOB: ${{ inputs.paths }}
         SOURCE_ROOTS: ${{ inputs.source-roots }}
         COMPOSER_PROJECT_PATH: ${{ steps.project-deps.outputs.project-path }}