Fix shallow merge-base handling for component fixtures

Increase the history depth to 150 and skip screenshot comparison with an explicit warning if the merge base is still unavailable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 7defd909-5470-4f45-97dc-09706d9c6570

Author: hediet

.github/workflows/component-fixtures.yml

@@ -30,8 +30,8 @@ jobs:
         with:
           # Need enough history for the merge-base lookup below to succeed even
           # when the target branch has advanced since the PR was opened. Full
-          # clone would be wasteful for this large repo, so cap at 50.
-          fetch-depth: 50
+          # clone would be wasteful for this large repo, so cap at 150.
+          fetch-depth: 150
 
       - name: Setup Node.js
         uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
@@ -184,8 +184,11 @@ jobs:
           if [ "${{ github.event_name }}" = "pull_request" ]; then
             # For PRs, diff against the merge-base with the target branch.
             TARGET_REF="origin/$BASE_REF"
-            git fetch --no-tags --depth=50 origin "$BASE_REF"
-            BASE_SHA=$(git merge-base "$EVENT_SHA" "$TARGET_REF")
+            git fetch --no-tags --depth=150 origin "$BASE_REF"
+            if ! BASE_SHA=$(git merge-base "$EVENT_SHA" "$TARGET_REF"); then
+              echo "::warning::Unable to find a merge base between $EVENT_SHA and $TARGET_REF. The depth-150 shallow history may not contain their common ancestor; skipping screenshot comparison."
+              exit 0
+            fi
           else
             # For push events, diff against the parent commit.
             BASE_SHA=$(git rev-parse "$EVENT_SHA^")
@@ -229,6 +232,7 @@ jobs:
 
       - name: Fetch base commit manifest
         id: base_manifest
+        if: steps.base.outputs.base_sha != ''
         env:
           BASE_SHA: ${{ steps.base.outputs.base_sha }}
         run: |
@@ -250,7 +254,7 @@ jobs:
 
       - name: Diff screenshots
         id: diff
-        if: always()
+        if: always() && steps.base.outputs.base_sha != ''
         run: |
           node build/lib/screenshotDiffReport.ts \
             https://hediet-screenshots.azurewebsites.net \