ci: detect new spec files against the merged base, not the stale PR base

pull_request.base.sha is fixed when the PR opens and does not follow the base branch, so a long-lived branch attributes every spec the base gained since then to itself. HEAD is refs/pull/N/merge, whose first parent is the base tip actually merged onto.

Author: christian-byrne

.github/workflows/ci-tests-e2e.yaml

@@ -268,8 +268,16 @@ jobs:
         id: detect
         run: |
           git config --global --add safe.directory "$GITHUB_WORKSPACE"
-          BASE_SHA="${{ github.event.pull_request.base.sha }}"
-          NEW_FILES=$(git diff --name-only --diff-filter=A "$BASE_SHA"...HEAD -- browser_tests/tests | grep '\.spec\.ts$' || true)
+          # HEAD is refs/pull/N/merge, so its first parent is the base tip the PR was merged
+          # onto. pull_request.base.sha is recorded when the PR opens and does not follow the
+          # base branch, so diffing against it attributes every spec the base gained since
+          # then to this PR.
+          if git rev-parse --verify --quiet HEAD^2 >/dev/null; then
+            BASE_SHA=$(git rev-parse HEAD^1)
+          else
+            BASE_SHA="${{ github.event.pull_request.base.sha }}"
+          fi
+          NEW_FILES=$(git diff --name-only --diff-filter=A "$BASE_SHA" HEAD -- browser_tests/tests | grep '\.spec\.ts$' || true)
 
           if [ -z "$NEW_FILES" ]; then
             echo "has-new-tests=false" >> "$GITHUB_OUTPUT"