chore/ci: Update the DiffTour workflow to match the shared action README

.github/workflows/diff-tour.yml

@@ -1,26 +1,27 @@
-name: Diff Tour link
+# This GitHub Action imported from the shared action
+# https://github.com/sourcegraph/actions/blob/main/diff-tour/
+# Please make fixes / enhancements in the shared action,
+# so all of our repos get the benefits
 
-# Posts a comment on each pull request linking to the Sourcegraph Diff Tour
-# for the changes in that PR, and keeps the comment up to date as the PR
-# changes. Once the PR is merged, the link points at the merge commit.
-#
-# Pull requests from forks are skipped: their GITHUB_TOKEN is read-only, so
-# the comment cannot be posted.
+name: Diff Tour link
 
 on:
   pull_request:
-    types: [opened, synchronize, reopened, closed]
+    # `edited` fires when the base branch changes; `synchronize` does not
+    # `closed` fires on merge and on plain close; either way the comment is
+    # updated to a link that survives deleting the head branch
+    types: [closed, edited, opened, reopened, synchronize]
 
+# The only permission needed. The action does not check out your repo, so
+# `contents: read` is not required, even for private repos
 permissions:
-  contents: read
   pull-requests: write
 
 jobs:
-  diff-tour-link:
-    name: Comment the Diff Tour link on the pull request
+  diff-tour:
+    name: Comment
     runs-on: ubuntu-latest
-    if: >
-      github.event.pull_request.head.repo.full_name == github.repository
-      && (github.event.action != 'closed' || github.event.pull_request.merged)
+    # Diff Tour resolves branch names against the base repo, so skip fork PRs
+    if: github.event.pull_request.head.repo.full_name == github.repository
     steps:
       - uses: sourcegraph/actions/diff-tour@main